Year: 1873



 In the first place, at the very outset

 says Prof. Agassiz in late lecture, we

 find one singular, universal fact. All

 animals, without exception, high or low,

 of whatever ultimate complexity ol

 simplicity of structure, originate from

 eggs, and from eggs of the same char

 acter. do not mean to be understood

 that there is no other mod0of multi

 plying. Some animals, as corals, for

 instance, many hydroids, and other low

 organisms, reproduce themselves by

 budding or by division of the parent

 stock. But they also, at some time or

 other, produce eggs, and thus bear their

 testimony to the general law of nature

 which applies to the whole animal king

 dom without exception. Indeed, the

 seed in plants has the same structure as

 the so-called ovarian egg in animals,

 and thus we may speak of all organized

 belngs, vegetable or animal, as multi plying by eggs. The discovery that this great law applies to the higher ami mals also, is comparatively recent. It IS only 50 years since Carl Ernst von Baer made his wonderful researches on the egg of the dog and rabbit, and an- | nounced that the mammal has an ovarian egg identical with that of all other yer tebrates and with that of articulates, mollusks and radiates. The discovery of this universal law that all organized beings multiply by the same means, is certainly one of the greatest and most startling generalizations of modern times. It gives us the broadest ground for our inquiry into their ultimate rela- tions since they have all one starting point. At first it may seem to simplify the inquiry. But the moment we leave the point at which all animals seem alike, and enter upon the various phases and transformations to be gone through before they reach the adult condition, field of study opens before us suffi- cient to fill many ifetimes. We come upon 8 series of phenomena of the deepest interest, but often in their first aspect very perplexing. There are transient phazes in the growth of the higher animals during which they re- semble the permanent final conditions of adult animals lower down in the same type, having, that is, a like struc- ture, but of inferior development. These comparisons cannot, however, be carried on outside the limits of one and the same type. To this recommendation of the law I alluded when, in the earlier part of my lecture, spoke of imita- tlons in the natural fundamental rela tlons of animals. You may compare a quadruped in certain phases of it's growth with the adult condition of some lower kinds of vertebrates and be amazed at the resemblance, but you can not carry the comparison over into the type of articulates or into any other type of the animal kingdom based upon a different plan. Within each type the | development has character as distinct | as the plan on which the type is built. An insect, for instance, can never at any time of it's development, after it has passed out of that universal condition of the ovarian egg to which I alluded. be compared to an oyster or a fish, but | 1t passes through phases where it can hardly be distinguished from a worm that IS, in the course of it's development 1t bears transient likeness to the adult condition of being a standing lower in the type of articulates to which they both belong. In short, every animal belonging to any one of the higher groups, during the transformations by which he reaches the adult state may pass through modified conditions, in each of which he resembles some being of his own type of the animal kingdom for whom that condition is final. Enough s known of the transformations of ami nals now to make this generalization a perfectly trustworthy one, although the number of illustrations is small. The judy of embryology is exceedingly hficult, and the sacrifice of specimens s very great in order to obtain the com 'lete history of the growth of any one nlmal. The progress of these investi- cations must necessarily be slow, be- ause theya are solaborious and so costly, ut they have already assumed sequence nd coherence enough to afford a sound .asis for the result I have just given. [ While the phases of growth and resem- . dance have been followed, the imita- ions keeping each primary type within I Is own boundaries have not been sufi- | , iently considered, and are, indeed, wholly lost sight of. This arelessness of investigation or of state- aent gives currency to broad generali- ations, partially tr1e indeed, but false 7hen they are made to cover too wide round.