# Natural History Review/Series 2/Volume 1/Number 2/General Results of the Study of Typical Forms of Foraminifera

Having been for some time engaged in the study of a series of typical representatives of several of the chief natural divisions of the Foraminifera, and finding that the general results of my inquiries are fully borne out by the study of other types prosecuted on the like method by Messrs. Rupert Jones and W. K. Parker, I think it desirable to draw the attention of naturalists to them, not merely as fixing the principles ​which must be taken as a guide in any attempt to frame a natural classification of that group, but as having an important bearing upon some of those higher questions relating to the origin and value of differential characters generally, which have recently been brought prominently under consideration. In so doing, it is my desire to confine myself purely to the scientific and practical aspect of these questions; seeking in the first place to determine, on the legitimate basis of induction, what general principles may be educed from the comparison of the large body of facts which I have brought together as regards the classification of Foraminifera; and then briefly inquiring how far the results of similar comparisons, made upon other types of organized structure, justify the extension of the same principles to the Animal and Vegetable Kingdoms at large.
It may be well for me to advert in limine to certain peculiar features in this inquiry, that render the group to which it relates singularly adapted for a comparison at once minute and comprehensive amongst a wide range of individual forms. The size of the greater part of these organisms is so small, that many hundreds, thousands, or even tens of thousands of them, may be contained in a pill-box; and yet it is usually not too minute to prevent the practised observer from distinguishing the most important peculiarities of each individual by a hand-magnifier alone, or from dealing with it separately by a very simple kind of manipulation. Hence the systematist can easily select and arrange in series such of his specimens as display sufficient mutual conformity, whilst he sets apart such as are transitional or osculant; and an extensive range of varieties may thus be displayed within so small a compass, that the most divergent and the connecting forms are all recognizable nearly in the same glance. I am not acquainted with any other group of natural objects in which such ready comparison of great numbers of individuals can be made; and I am much mistaken if there be a single species of plant or animal, of which the range of variations has been studied by the collocation and comparison under one survey of so large an assemblage of specimens as have passed under review in the course of these investigations.
The general fact which I desire to bring prominently forward as the result of my investigations into this group, is, that in all the types possessing a wide geographical distribution which have been specially studied by myself, or by others, the range of variation has also been very wide; so that not only what have been considered as specific, but such as have been regarded as generic, and in some cases even as ordinal differences, present themselves among organisms, which, from the intimacy of the mutual relationship that is evinced by the gradational character of those differences, as well as by the variation presented in the several parts of one and the same organism, must in all probability have had a common origin.
​And it appears to me a justifiable inference from this fact, that the wide range of forms which this group contains, is more likely to have come into existence as a result of modifications successively occurring in the course of descent from a small number of original types, than by the vast numbers of originally distinct creations which on the ordinary hypothesis would be required to account for it.
The greater part of my first memoir was devoted to the investigation of a single type, Orbitolites; and I there showed, that not only as regards the size, shape, and other external characters of the organism as a whole, but even as regards the size and form of its elementary parts, in which greater constancy might be expected, is there so great a variation (the most marked diversities being apparent even in different parts of the same specimen), that all attempts to found specific distinctions upon such variations are utterly futile. But further, I showed that a distinction on which almost any naturalist would feel justified in relying, as of specific if not of generic value, that between the simple type in which all the cells are arranged on only one plane, and the complex type in which there are two superficial planes more or less strongly differentiated from the median, is no less invalid. For although these types are usually distinguishable the one from the other without the least difficulty, yet they are often combined in the same individuals, and this in such a variety of modes, that the transition from the simple to the complex may be clearly seen, by the comparison of a sufficient number of specimens, to be by no means attributable to a mere advance of age. Further, having been furnished (by the kindness of Mr. H. J. Carter) with specimens of the Scindian fossil which presents the characters ascribed by M. D'Orbigny to his genus Cyclolina, I find, as I had anticipated, that this genus is founded upon a mere variety of Orbitolites, in which the character of the surface-marking is more than ordinarily cyclical. Not merely, however, does the range of variation of this type confound the ordinary distinctions of systematists in regard to species and genera; it extends also to that difference in plan of growth which has been assumed by M. D'Orbigny of such fundamental importance, as justly to constitute the essential difference between his two orders Cyclostègues and Helicostègues. For, as I have shown, although Orbitolites is typically cyclical from its commencement, yet specimens frequently present themselves in which its early development has taken place so completely on ​the helical plan, that if such had been collected before their assumption of the cyclical mode of growth, their essentially Cyclostègue character would not have been suspected.
Again, I have shown (2nd series) that a parallel variation is displayed by the genus Orbiculina, whose ordinarily helical plan of growth has caused M. D'Orbigny to range it among his Helicostègues, notwithstanding that in fully developed specimens its mode of growth is not unfrequently cyclical. The occasional exchange in this type of one plan of increase for the other, at an advanced period of life, is a fact of very high interest; for when an Orbiculina has undergone this change, the outer or cyclical portion of its disk can in no way be distinguished from that of Orbitolites; and the only difference between these two types which has any semblance of validity is the absence in Orbitolites of those successive encasings of the central nucleus, the presence of which seems to be a constant feature in Orbiculina.
It is to be observed, however, that these successive encasings are due entirely to the extension of the later whorls of the spire over the earlier, and that they are no longer found in Orbiculina when the helical mode of growth gives place to the cyclical. Hence it seems not unfair to surmise that if the helical growth of an aberrant Orbitolites were to continue until its spire had made several turns, instead of stopping before the completion of one, its nucleus would receive successive investments from successive whorls, just as in the typical Orbiculina, and the only difference between these two types would thus vanish.
On the other hand, if the helical growth of an Orbiculina were to give place to the cyclical at an unusually early period, the central nucleus would receive no investment, and would present the flatness by which that of Orbitolites is characterised as compared with that of the typical Orbiculina. Hence the idea of the derivation of Orbitolites and Orbiculina from the same original must be admitted to be scarcely less probable than that of the derivation of the helical and the cyclical forms of Orbiculina, or of the simple and complex types of Orbitolites, from a common parentage.
Let us now apply the same mode of inquiry to Alveolina. I have shown (2nd series) that this organism is closely allied in every other respect than its geometrical plan of growth to the types we have just been considering; the structure of the shell and its relations to the contained body, and the relations of the segments of that body to each other, and to the external world, being essentially the same in them all.
Now however improbable it may seem at first sight that an Orbitolites, which extends itself as a flat or bi-concave disk by successive concentric growths, and an Alveolina acquiring a fusiform shape by successive turns round a progressively elongating axis, should have a common original, yet, when the intermediate links are duly studied, a continuous gradation is found to be established. For, as has just been shown, a longer continuance of the helical mode of growth in which Orbitolites often commences, would really produce an Orbiculina, with its centre so invested by successive whorls as to form a vertical ​linear axis; and we find this axis in Orbiculina, sometimes equalling in length the diameter of the spire, so that this organism at an early stage of its growth may be nearly spheroidal. Now among the various types of fossil Alveolina, there are some whose shape, instead of being fusi-form, like that of the recent type I have described, is almost identical with that of a spheroidal Orliculina; and the general structure of two such organisms will be so nearly identical, that I cannot see any difficulty in referring them to a common original. And when we examine a series of such fossil types, we see that the}' present a wider and wider divarication from the Orliculina type in this one particular alone, that whilst the later growth of Orliculina tends to liken it to Orbitolites, that of Alveolina tends to the continual elongation of its vertical axis—a difference which all analogy would indicate to be one of far too small account in this group to be justly taken as a ground of original distinction.
In the assemblage of forms which I have thought myself justified in re-assembling under the designation Peneroplis (3rd series), we encounter other remarkable series of variations, the principal of which have given occasion to the formation of the two additional genera Dendritina and Spirolina. With an exceedingly close conformity in the texture and in the superficial markings of their shells, as well as in their general plan of growth, we observe a marked diversity in the form and proportions of the spire, especially in the later stages of its growth, and a still greater divergence in regard to the form and disposition of the septal apertures. For in the type to which M. D'Orbigny restricts the generic designation Peneroplis, we usually find the spire rapidly widening and becoming proportionally compressed in each succeeding convolution; whilst in that which he distinguished as Dendritina, the spire widens but slowly whilst increasing rapidly in turgidity. Further, in the one type, as in the other, the later extension is often in a straight line, instead of continuing to follow the spiral course; and on this variation alone, which (as will presently appear) is of no account whatever among Foraminifera, has been erected the genus Spirolina. Now, in the typical Peneroplis, the septal plane presents a single linear series of minute rounded pores, whilst in the typical Dendritina we find in their place a single large orifice with radiating extensions, the difference between these two modes of communication being as great as we find between al- most any two types of Foraminifera whatever. Yet I believe that no one who will go through the details of the evidence I have collected from the study of transitional forms, will have any doubt that Peneroplis and Dendritina may have had a common progenitor, and that the peculiarity in the mode of septal communication that characterises each is intimately related to the compressed or turgid form of the spire in each case; whilst the different forms of Spirolina type, among which we find the most remarkable transitional conditions of aperture, are so obviously related to one or other of the foregoing, that no reasonable doubt can exist of their derivation from these. Now, the geographical distribution of the two fundamental types is so far different, that where one prevails, the other ​is either absent altogether, or presents itself under a modified form; and thus we seem justified in the belief that whether either has been derived from the other, or both have been derived from some intermediate form (such as that which seems common alike to the young of both), the modifications which have given rise to the marked differences they now exhibit, are mainly due to diversities in the external conditions under which they have been respectively propagated.
But to what other type does Peneroplis itself present the closest approximation? By systematists in general, the intimate relationship which I have shown it to possess to the helical type of Orbiculina has been so slightly regarded, that it has been considered as at least equally related to the Operculina type; and yet, as I shall presently show, these two types are removed from each other in all the most essential features of their structure, as far as any two polythalamous Foraminifera can be. And the idea of the derivation of Peneroplis from the same stock with Orbiculina seems justified by the fact that the young forms of the two are frequently so alike as not to be distinguishable by external characters alone, whilst their internal difference consists only in the presence or the absence of the secondary or transverse septa—a character which I have shown reason to regard as variable in this group.
Notwithstanding, therefore, the apparently wide divergence of the cyclical Orbitolites, the helical Orbiculina, the fusiform Alveolina, and the simply-chambered Peneroplis and Dendritina, these several types must be regarded as most intimately related to one another; and that relationship seems to me much more likely to have arisen from a common ancestral descent, than from the original creation of independent types, capable of graduating into each other so continuously as almost to assume each other's characters.
It is very important to remark that they all possess that peculiar texture of shell, which is designated by Professor Williamson as porcellanous; presenting an opaque white hue when seen by reflected light, but a rich brown or amber colour, when seen by light transmitted through thin natural lamellæ or artificial sections. This substance is entirely structureless, and possesses no great density or tenacity. Moreover, in all the foregoing types, each of the septa intervening between the chambers consists of only a single layer; and the passages of communication between them are, for the most part, so large and free, that the segments of the sarcode-body are but very imperfectly isolated from each other; and, as might be anticipated from this incompleteness of separation, it is here that variations in the mode of communication between the chambers seem to be of least account. It is in this type that we re​cognize the nearest approximation towards such forms as Thalassicolla, which seem to connect Orbitolites with Sponges; while the relationship which Orbiculina and Peneroplis have been supposed to bear to the ordinary Helicostègues, being dependent only on plan of growth, and being utterly at variance with the essential characters of the two groups, must be regarded as one of analogy, not affinity. Looking to the evidence I have adduced in regard to the prevalence of particular modifications of Orbitolites in particular localities, and to the influence of the geographical distribution of the Peneroplis type upon the modifications it presents, we seem justified in extending the same view to those larger (though not more essential), differentiations which these types must have undergone on the hypothesis of their derivation from the same original. The following may be suggested as the mode in which the existing forms might thus have diverged from each other, and from their primary type.
ORBICULINA TYPE, :
| Diverging into |||
| $\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } }$ |||
| Peneroplis. | Orbiculina. |
| $\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad } }$ | $\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad } }$ |
| Dendritina, Peneroplis, Dendritina, Spirolina, Peneroplis. | Alveolina, Orbiculina, Alveolina, Orbiculina, Orbitolites. |
Passing on, now, to an essentially different group, of which Operculina may be taken as the type, I have shown that the relation of the discoidal Cycloclypeus and the helicene Heterostegina is of essentially the same nature with that of Orbitolites and Orbiculina; the minute structure of the shell and the physiological condition of the sarcode body being essentially the same in the two organisms, and the only important divergence between them being in their plan of growth. Prom the rarity of Cycloclypeus, all the specimens of which yet known have been brought from one locality, I have not yet had the opportunity of ascertaining whether it ever presents in an early stage any approximation to the helical mode of growth; but such a deficiency of affirmative evidence is obviously not equivalent to a disproof of what has strong analogy in its favour.
The variations which I have described (3rd series) among the different forms of Operculina, although limited to the form of the spire, and the character of the surface-markings, would be amply sufficient to justify the erection of numerous species, were it not for the connexion established between the most divergent forms by intermediate links, and the necessity for an almost indefinite multiplication of hypothetical originals which the adoption of such a method would involve. The existence of such a large extent of variation among the specimens collected in the same locality must be admitted as valid evidence of the facility with which differential characters develope themselves in this type; and a strong probability is thus afforded in favour of the varietal character of larger ​differences among individuals whose conditions of existence are very diverse. Hence the analogy of Operculina affords good grounds to surmise that many of the reputed species in the nearly-allied genus Nummulites have no real title to that rant; the differences among many of them being not nearly so great as those we have met with among the varieties of Operculina; whilst those presented by many others do not exceed what might be reasonably expected to occur under a greater variety of modifying agencies. But I have shown that it may be fairly questioned whether there is adequate ground for upholding the generic distinctness of Operculina and Nummiilites; the characteristic by which the latter has been asserted to be specially distinguished, being not unfrequently observable as a varietal difference in the former. The form which I have described under the designation of Amphistegina Cumingii, bears a striking resemblance to the ordinary Nummuline type in the early part of its growth, and to the ordinary Operculine in the later; and may be regarded as in many respects a connecting link between the two.
There appears, then, strong reason for considering Cycloclypeus, Heterostegina, Operculina, Nummulites, and Amphistegina as related to each other in the same manner and degree as the leading forms already enumerated under the Orbiculine group. And it is very curious to observe the perfect analogy which prevails in regard to the forms under which these two great types of structure—essentially different as they are—tend to develope themselves. As I have already pointed out, the relation of Cycloclypeus to Heterostegina is exactly that of Orbitolites to Orbiculina. So, if the transverse or secondary septa of Heterostegina were undeveloped, we should have an Operculina, Nummulina, or Amphistegina (these three types being, in my view, essentially one and the same), just as the like deficiency actually occurring in Orbiculina gives to it all the essential characters of Peneroplis. And the parallelism seems to be completed by the existence in Fusulina of the same metamorphic condition of this type, that Alveolina is of the Orbiculine. The accordance of all these in the highly elaborated texture of the shell, in the relation which this bears to the segments of the sarcode-body, and in the presence of an intermediate skeleton with its canal system, is extremely close. The substance of the shell is very dense, and of almost vitreous transparence where it is not perforated by the minute closely-set tubuli, which usually pass direct from the interior of the chambers towards the external surface. Each segment of the body has its own proper envelope, so that the septa between the chambers are composed ​of two distinct laminæ, which diverge from each other where they give passage to the canal system, and which are often further separated by the intervention of a portion of the intermediate skeleton. The passages of communication between the chambers are so narrow, that segments of the body are much more isolated from each other than they are in the type already described; and the proper walls of the chambers seem, as it were, to be moulded upon the segments, instead of merely filling-up the interspaces, between them, as they there do. This filling up, in fact, is the office of the intermediate skeleton, which gives a solidity to the whole aggregation that it would otherwise want; and special provision, as we have seen, is made in the canal system for its nutrition. Altogether this type is the one in which the Foraminiferous structure attains its highest development, and which is most completely differentiated from every other. And the morphological variations it is known to undergo seem to me fully to justify the inference that such further variations as have been shown to occur in the Orbiculine type might be regarded as the probable source of the divergence from some common ancestral stock of the several forms whose intimate relationship I have demonstrated. The analogy of that type would suggest Heterostegina as presenting the nearest existing approximation to such a common original; and the stages of differentiation may be thus expressed:—
HETEROSTEGINE TYPE, :
| Diverging into |||
| $\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } }$ |||
| Orbiculina. | Heterostegina. |
| $\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } }$ | $\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } }$ |
| Amphistegina, Nummulites, Operculina. | Heterostegina, Cycoclypeus. |
From my imperfect acquaintance with Fusulina, I do not feel justified in expressing its exact relationship to either of the forms included in this scheme; and, for the same reason, I abstain from connecting Orbitoides with Cycloclypeus, to which it has some features of close relationship.
After this detailed examination of the general relations of the principal modifications of two of the most strongly-marked types to be found in the whole group of Foraminifera, it seems needless for me to do more with respect to the other forms whose structure I have investigated, than to inquire how far the peculiar characters by which they are respectively distinguished show evidence of a like variability. Thus I have shown (4th series) that Calcarina is essentially distinguished from Rotalia by the extraordinary development of the intermediate or supple​mental skeleton, and by the extension of this into radiating prolongations. But the number, forms, and proportions of these prolongations are subject to very considerable variations; so that, whilst they are sometimes so greatly multiplied and prolonged as to constitute the principal feature of the organism, they are so little developed in other instances, that the contour of the disk is scarcely interrupted by them. Further, I have shown that the development of this supplemental skeleton is, in a great degree, independent of that of the spire; hence, if this last be the essential component of the organism (as all analogy indicates), the supplemental skeleton must be regarded as a feature of minor importance. On the other hand, the development of radiating out-growths is an occurrence not unfrequent among other helicine Foraminifera, even in species whose typical form is altogether destitute of them (as Professor Williamson has pointed out in Polystomella crispa); and such forms differ much less widely, as regards this character, from the simpler forms of Calcarina, than these last do from the very complex forms with which they are connected by a continuously-gradational series. Hence, I cannot regard the remarkable development of the supplemental skeleton in Calcarina as affording any disproof of its genetic relationship to Rotalia, with which its affinity in every other particular is most intimate.
If, again, we inquire into the import of that remarkable development of the canal-system, which seems to be the distinctive feature of Polystomella (4th series), we find that if we base our judgment upon a sufficiently wide foundation of facts, its non-essential character becomes apparent. For although the large P. craticulata of the tropical and Australian seas presents the most symmetrical and extensive distribution of the canal-system that I have anywhere met with, the little P. crispa of our own seas exhibits but feeble traces of it; yet of the intimacy of their relationship no doubt can be fairly entertained. I have shown (3rd series) that a parallel difference exists between the gigantic Amphistegina Cumingii and the comparatively diminutive A. gibbosa; as also (4th series) between the two forms of Tinoporus, where its presence or absence is obviously associated with the presence or absence of the radiating prolongations, and of the supplemental skeletons from which these proceed.
In considering the import of the canal-system as a character for the systematist, the mode of its formation must not be left out of view. I have shown that the passages which altogether go to make up this system are not true vessels, but are mere sinuses, left in some cases by the incomplete adhesion of the two contiguous walls which separate adjacent chambers, and in other cases apparently formed by the incomplete calcification of the sarcode which forms the basis of the solid skeleton; certain portions of that substance remaining in their original condition, so as to maintain a communication between the contents of the chambers and the parts of the shelly casing most removed from them, just as the fissures or pores which communicate between the chambers, and between the last chamber and the exterior, are mere unconsolidated portions of the septa, occupied in the living state by commissural por​tions of the sarcode body. Hence it is readily conceivable how a canal-system may be formed with considerable regularity in an organism in which the intermediate skeleton attains a considerable development, whilst it may be wholly or partially deficient in another, in which that supplemental deposit of calcareous matter has taken place to a much smaller extent. And it is to be specially observed that all those forms in which it is at present known to attain its greatest completeness, are those tropical or semi-tropical types, in which the influence of warmth, abundance of food, and other external agencies in promoting development, appear specially to favour the largest growth and the most specialized evolution of the Foraminiferous type.
The relations of the forms belonging to the family Miliolitidæ have recently been investigated by Mr. W. K. Parker; and his results are in perfect accordance with my own. Thus in each of the genera Cornuspira, Hauerina, and Vertebralina, Mr. Parker reduces all the reputed species to one; while he shows that even their generic differences are really but of small account. And he not only in like manner reduces all the reputed species of the genus Milola to the level of varieties, but brings down to the same rank the reputed genera Spiroloculina, Biloculina, Triloculina, and Quinqueloculina; the differences between which, arising from asymmetrical growth, and from variations in the form and number of the chambers, cannot be regarded as even of specific value, the Milioline plan of construction being preserved throughout. In the large group of Nodosarinæ which has been carefully studied by Messrs. T. Rupert Jones and W. K. Parker, those gentlemen have felt themselves justified, on the like grounds in reducing a multitude of reputed genera and species to a single type. Between the nautiloid Cristellariæ and the straight moniliform or rod-like Nodosariæ, which agree in essential characters of structure and mode of growth, they find such a continuous series of connecting links, that no line of demarcation can be anywhere drawn, the straight, the curved, and the spiral forms passing gradationally one towards another; and the extreme forms being thus brought together, the various intermediate grades which have been distinguished by systematists under the generic names Glandulina, Lingulina, Dentalma, Rimulina, Vaginulina, Planularia, Marginulina, Dimorphina, Flabellina, and Frondicularia, necessarily fall into the same category.
The same general doctrine having thus been shown to hold good in regard to all the chief natural subdivisions of the Foraminiferous group, it is not my purpose at present to prolong the inquiry in this direction. The systematic study of this tribe needs to be prosecuted far more extensively than my own time and opportunities have admitted, to enable even an outline scheme to be framed, which should represent an approach to the true relations of its principal families. But I think I ​have made it clear that such a scheme will be most likely to approach the truth, when its basis is laid in a thorough knowledge of the nature and extent of those variations which every chief modification of this type shows itself so peculiarly disposed to exhibit, and when in building it up the idea of natural affinity is accepted as expressing not only degree of mutual conformity, but actual relationship arising from community of descent more or less remote. For the endless gradational departures from any types which we may assume as fixed, and the occurrence of links of connexion between such as present the best marked differentiations, seem to me to point unmistakeably to this as the only escape from that difficulty of indefinite multiplication, which attends the application of the doctrine of distinct specific creations to a group in which scarcely any two individuals are alike.
The present aspect of this inquiry, in fact, may be not inappropriately compared with that of the oft-debated question as to the Races of Mankind. In the one case, as in the other, the direct evidence of descent affords cogent evidence as to the possible extent of modification within the limits of particular races; and when that evidence is brought into relation with analogous facts in regard to the yet greater variations of which we have direct evidence in the case of domesticated animals, it points to conclusions of higher generality, which physiologists find no difficulty in accepting. Now the modifications which any single type of Foraminifera must have undergone, to give origin to the whole series of diversified forms now presented by that group, are not greater in comparison with the modifications of which we have direct evidence, than are those which the advocate for the specific unity of the human races has no hesitation in assuming as the probable account of their present divergence.
This view of the case derives great force from the fact that there is strong reason to regard a large proportion of the existing Foraminifera as the direct lineal descendants of those of very ancient geological periods—a doctrine first advanced by Professor Ehrenberg in regard to a considerable number of Cretaceous forms, and since fully confirmed and extended as regards the Tertiary fauna by the admirable researches of Messrs. Rupert Jones and Parker, as well as by my own comparison of the recent and fossil types of Orbitolites, Orhiculina, Alveolina, Operculina, and Calcarina; and shown to be applicable also to the Secondary fauna, as far back as the Triassic system, by the remarkable results of the investigations of the same gentlemen in regard to a well-preserved sample of it. Following out, by laborious and extended comparison, the method of inquiry I have so much insisted on, they have found ample evidence that a like range has prevailed through the whole succession of geological periods to which their researches have extended. "Our own experience of the wide limits within which any specific group of the Foraminifera multiply their varietal forms, related by some peculiar conditions of growth and ornamentation, has led us to concur fully with those who regard nearly every species of Foraminifera as capable of adapting itself, with endless modifications of form and structure, ​to very different habitats in brackish and in salt water; in the several zones of shallow, deep, and abyssal seas; and under every climate, from the poles to the equator. In arranging our synoptical tables of the Mediterranean Rhizopoda, recent and fossil, and in comparing their numerous specific and varietal forms one with another, we have not confined ourselves to our collections from this region, but have necessarily made comparisons of forms from almost every part of the globe; from the Arctic and the Tropic Seas; from the temperate zones of both hemispheres; and from shallow, as well as deep sea-beds. Geologically, also, we have reviewed the Foraminifera in their manifold aspects, as presented by the ancient Faunas of the Tertiary, Cretaceous, Oolitic, Liassic, Triassic, Permian, and Carboniferous times; finding, to our astonishment, that scarcely any of the species of Foraminifera met with in the Secondary Rocks have become extinct; all, indeed, that we have yet seen have their counterparts in the recent Mediterranean deposits. This is still more clearly found to be the case with regard to the Chalk of Maestricht and the Tertiaries". And the same excellent observers, in summing up their description of the Foraminifera of the blue clay met with in the alabaster pits at Chellaston, near Derby, belonging to the Upper Triassic series, thus express themselves:—"Having thus pointed out that, judging from these specimens obtained at Chellaston, the minute Nodosarinæ and other Foraminifera of the Triassic period have continued to exist through the intermediate ages to the present day, without losing any of their essentially specific features, we will observe that the Nodosariæ are present in rocks of still greater age than the Trias,—namely, the Permian and Carboniferous, and probably even the lower Silurian. Nodosariæ and Dentalinæ abound in some of the Permian limestones of Durham and the Wetterau, in company with Textulariæ. Nodosaria occurs also in the Carboniferous limestone of Ireland, according to M'Coy; and the green sand of the lower Silurian series, near St. Petersburg, has yielded to Ehrenberg casts of chambers something like those of Dentalina, together with unmistakeable casts of Testularian and Rotalian shells. We may remark, too, that the Fusulina of the Russian, North American, and Arctic Mountain-limestone carries back the pedigree of the Nonionina group to the Palæozoic periods; and that it is accompanied with other Foraminifera of known types, amongst which Nummulina is not absent. This last-named type has rare representatives in the Lias and Oolite; it acquired great potency in the Tertiary seas, and is not extinct now. Altogether we have here some remarkable instances of the persistency of life-types among the lower animals. Though the specific relations of the Palæozoic Foraminifera require further elucidation, we feel certain that the six genera, represented in this Upper Triassic clay of Chellaston by about ​thirty varieties, stand really in the place of ancestral representatives of certain existing Foraminifera, that they put on their several subspecific features in accordance with the conditions of their place of growth, just as their posterity now do; and that although we have in this instance met with only the minute forms of a 700-fathoms mud-bottom, yet elsewhere the contemporaneous fuller development of these specific types may be found by careful search in other and shallower deposits of the Trias period".
It can scarcely, I think, be questioned that such a continuity of the leading types of Foraminifera maintained through so long a series of geological periods, and the recurrence of similar varietal departures from those types, is a result of the facility with which creatures of such low and indefinite organization adapt themselves to a great variety of external conditions; so that, on the one hand, they pass unharmed through changes in those conditions which are fatal to beings of higher structure and more specialized constitution; whilst on the other, they undergo such modifications, under the influence of those changes, as may produce a very wide departure from the original type.
Thus we have found strong reason for regarding temperature as exerting a most important influence in favouring, not merely increase of size, but specialization of development; all the most complicated and specialized forms at present known being natives either of tropical or of sub-tropical seas, and many of these being represented in the seas of colder regions by comparatively insignificant examples, which there seems adequate reason for regarding as of the same specific types with the tropical forms, even though deficient in some of their apparently most important features. The depth of the sea-bottom seems also to affect the prevalence of particular types, and to modify the forms under which they present themselves; so that Messrs. Jones and Parker feel themselves able to pronounce approximately as to the depth of water at which a deposit of fossil Foraminifera may have been formed, by a comparison of its specific and varietal types with those characterizing various depths at the present time. And it is specially worthy of note, that in the greatest depths of the ocean from which Foraminifera have been brought by deep-sea soundings, these belong almost exclusively to one type, Globigerina.
Now it may be at once conceded that no other group in the Animal kingdom affords any thing like the same evidence, on the one hand, of the derivation of a vast multitude of distinguishable forms from a few primitive types, and on the other, of the continuity of those types through a vast succession of geological epochs. A somewhat parallel case, however, as regards the first of these points, is presented by certain of the humbler groups of the Vegetable kingdom, in which it is becoming more and more apparent, from the careful study of their life-history, not ​only that their range of variation is extremely wide, but that a large number of reputed genera and species have been erected upon no better foundation than that afforded by the transitory phases of types, hitherto known only in their states of more advanced development.
But it would be very unreasonable to put aside these cases as so far exceptional, that no inferences founded upon them can have any application to the higher forms of animal and vegetable life. For it is only in the degree of their range of variation, that Foraminifera and Protophyta differ from Vertebrata and Phanerogamia; and the main principle which must be taken as the basis of the systematic arrangement of the former groups,—that of ascertaining the range of variation by an extensive comparison of individual forms,—is one which finds its application in every department of Natural History, and is now recognized and acted on by all the most eminent botanists and zoologists. It will be sufficient for me here to refer to the views recently advanced by Dr. J. D. Hooker, in his introduction to the Flora of Australia; the results of his extensive experience in the comparison of the Floras of different portions of the globe having led him to conclusions regarding the probable origin of the diversities they present, with which my own deductions from the study of the Foraminifera are in complete accordance. And I am authorized by Mr. T. Davidson, whose profound knowledge of the Brachiopoda enables him to speak as the highest authority upon all that relates to that most interesting group (which, like that of Foraminifera, is traceable through the entire series of fossiliferous rocks) to state that in proportion to the increase of his knowledge of its modifications of type, does he find reason to regard many of them as possessing so wide a range of variation, that he feels justified in making a large reduction in the number of specific types hitherto accounted distinct; whilst in the same proportion he finds himself able to trace with considerable probability the same specific types through a succession of geological periods,—certain Oolitic Terebratulida, for example, being the probable ancestors of existing forms; and even the Lingula of the Wenlock Silurian being specifically undistinguishable from the Lingula anatena of our present seas.
The following are the general propositions, which it appears to me justifiable to base on the researches of which I now give a résumé:—
I. The range of variation is so great among Foraminifera, as to include not merely the differential characters which systematists proceeding upon the ordinary methods have accounted specific, but also those upon which the greater part of the genera of this group have been founded, and even in some instances those of its orders.
II. The ordinary notion of species, as assemblages of individuals marked out from each other by definite characters that have been genetically transmitted from original prototypes similarly distinguished, is ​quite inapplicable to this group; since even if the limits of such assemblages were extended so as to include what would elsewhere be accounted genera, they would still be found so intimately connected by gradational links, that definite lines of demarcation could not be drawn between them.
III. The only natural classification of the vast aggregate of diversified forms which this group contains, will be one which ranges them according to their mode and degree of divergence from a small number of principal family types; and any subordinate groupings of genera and species which may be adopted for the convenience of description and nomenclature, must be regarded merely as assemblages characterized by the nature and degree of the modifications of the original type which they may have respectively acquired in the course of genetic descent from a common ancestry.
IV. Even in regard to these family types, it may be fairly questioned whether analogical evidence does not rather favour the idea of their derivation from a common original, than that of their primitive distinctness.
V. The evidence in regard to the genetic continuity of the Foraminifera of successive geological periods and of those of the later of these and the existing inhabitants of our seas, is as complete as the nature of the case admits.
VI. There is no evidence of any fundamental modification or advance in the Foraminiferous type from the Palæozoic period to the present time. The most marked transition appears to have taken place between the Cretaceous period, whose Foraminiferous Fauna seems to have been chiefly composed of smaller and simpler types, and the commencement of the Tertiary, of which one of the earliest members was the Nummulitic limestone, which forms a stratum of enormous thickness, that ranges over wide areas in Europe, Asia, and America, and is chiefly composed of the largest and most specialized forms of the entire group. But these were not unrepresented in previous epochs; and their extraordinary development may have been simply due to the prevalence of conditions that specially favoured it. The Foraminiferous Fauna of our own seas probably presents a greater range of variety than existed at any preceding period; but there is no indication of any tendency to elevation towards a higher type.
VII. The general principles thus educed from the study of the Foraminifera should be followed in the investigation of the systematic affinities of each of those great types of animal and vegetable form, which is marked out by its physiological distinctness from the rest. In every one of these there is ample evidence of variability; and the limits of that variability have to be determined by a far more extended comparison than has been usually thought necessary, before the real relations of their different forms can be even approximately determined.
VIII. As it is the aim of the physical philosopher to determine ​"what are the fewest and simplest assumptions, which being granted, the whole existing order of nature would result," so the aim of the philosophic naturalist should be to determine how small a number of primitive types may be reasonably supposed to have given origin, by the ordinary course of "descent with modification," to the vast multitude of diversified forms that have peopled the globe in the long succession of geological ages, and constitute its present Fauna and Flora.
* ↑ See my Researches on Foraminifera, first and second Series, in the Philosophical Transactions for 1856; third Series, op. cit., 1859; fourth Series, op. cit., 1860.
* ↑ I have the authority of M. Deshayes for the belief that the excessive multiplication of generic and specific distinctions, which so greatly impairs the value of the late M. D'Orbigny's labours upon this group, was due to his having based these distinctions upon specimens selected for him as typical, and to his having disregarded the transitional forms which any large collection of these organisms is sure to contain in abundance; thus, to use the admirable discrimination of the Prince of Canino, "describing specimens rather than species."
* ↑ In order to avoid misapprehension, I would here remark that the production of any organism seems to me just as much to require the exertion of Divine Power, when it takes place in the ordinary course of generation, as it would do if that organism were to be called into existence de novo; the question being, in reality, whether that exertion takes place in the way of continuous exercise, according to a settled and a comprehensive plan, or by a succession of disconnected efforts.
* ↑ My statement on this point is fully confirmed by Messrs. Parker and Rupert Jones, who state that, not "unfrequently, feebly-developed Peneropliform varieties, as well as good-sized Adunciform specimens, occur, in which the long narrow chambers are at times simple and undivided, being occupied by transversely-elongate lobes of sarcode, instead of numerous minute, sub-cubical blocks." See Ann. of Nat. Hist., March, 1860, p. 180.
* ↑ It is questioned by Messrs. Parker and Rupert Jones whether this is a true Amphistegina, chiefly on account of its bilateral symmetry (Ann. of Nat. Hist., Feb., 1860, p. 111). But I have met with perfect bilateral symmetry in specimens warranted as Amphistegina by those excellent judges of that type.
* ↑ I have not yet been able to satisfy myself as to the precise affinities of Fusulina, the metamorphic condition of its shell interfering with the minute study of its structure; but my view of its nature essentially corresponds with that of Messrs. Parker and Rupert Jones. (See Quart. Journ. of Geol. Soc, Nov., 1860, p. 458.)
* ↑ The figure given by Prof. Ehrenberg, in his remarkable memoir already referred to, "Ueber den Grünsand und seine Erläuterung des organischen Lebens," Plate IV., fig. 8, and by him designated as the internal cast of Orbitoides javanicus, will be seen on comparison to present a most remarkable correspondence with figs. 10, 11, 12, of Plate XXIX., illustrating my description of Cycloclypeus.
* ↑ Transactions of the Microscopical Society for 1858 (New Series, vol. vi.), p. 53.
* ↑ Annals of Natural History, Nov., 1859 p. 477; and Quarterly Journal of the Geological Society, August, 1860, p. 302 and November, 1860, p 454.
* ↑ "On the Rhizopodal Fauna of the Mediterranean, compared with that of the Italian and some other Tertiary Deposits," in the Quarterly Journal of the Geological Society for August, 1860, p. 294.
* ↑ "On some Fossil Foraminifera from Chellaston, near Derby," in the Quarterly Journal of the Geological Society for November, 1860, p. 458.
* ↑ See especially on this subject the valuable researches of Dr. J. Braxton Hicks, "On the Development of the Gonidia of Lichens, in relation to the Unicellular Algæ," in Quart. Journ. of Micr. Science, October, 1860, and January, 1861.
* ↑ Mill's Logic, 3rd ed., vol. i., p. 327.