The sun. [volume] (New York [N.Y.]) 1833-1916, 1899-02-26
III.

The eleventh stage In the evolution OF man is
that OF tho cyclostomata, now exemplifled In
the small group Of lampreys and hagflshes.
representing the lowest c%raniota. Although
much specialized. as q side branch OF the main
stem from which the other craniota have
sprung. they give us an idea of what the direct
ancestors OF the latter must have been like:
still without visible arches, without Jaws and
without paired limbs with q persistent prone
phros: tho ear with one semi-circular canal
only. mouth suctorlal.: Uranium very prim-
tive. and the metamerism of the vertebral COL
umn indicated only by little blocks OF cartilage
in the perichordal sheath. such creatures
must have existed at least as early az the
Lower Silurian epoch but, until 1880, fossil cy-
clostomes were unknown. Their life il the mud.
or as endopurasitcs of fishes. coupled with their
soft structure. makes them very unfit for pres
e%rvation. The twelfth stage is that OF tho elasmo-
branchi tsharks and skates. which. with their
immediate fOrerunners, the acanthodi OF the
Devonian and Carboniferous age. are the first
typical fishes. That they existed as far back
as the Silurian age is proved by many en-
amelled spines Of the dermal armor. chiefly
from the dorsal fins. This twelfth stage is
characterized by the possession of typical jaws.
by visceral or Rill-bearing arches. and by two
pairs Of limbs. None of the elasmobranchi
indeed. whether fossil or recent. stands in the
direct ancestral line OF man hut they AFRO the
lowest .Gnathostomata., or jaw and limb
nected with the elasmobranchi in a wider
sense are the crosSopterygii. which begin in
f have HIe eft on!y sES s sn i isr fSe kff EGisPi P pSt
lypterus. and clamoich;hys. They are pos
sessed of dermnl bones aud other oSsifcations;
and are characterized by their lobate- paired
fms, which have a thick axis heset with biserial
fin rays. Their gill cleits are covered by an
more highly developed than the S"iasmnobranchi;
ganoids and other bony fishes all. OF which
form "A great manifold side braIch of the gen.
particulars inotahly the structure OF the paired
fins. the y%ertcbral column and the air bladderl
nearer TIP me il m EhaH,, stem be ibo on mans ,ims, 70 an pSia,,nn
to the dipnoi, which are still re presented by
the ,Anatralian, African and south American
lows. The paired fms still retain the archip-
tcrygial form namely. one axis. with biserial
c"Ivos blood which i. mixed arterial and
y,cnous, owing 70 the gills being retained while
III fact. the generalized dipnoi form the actual
link hetween fishes and amphlbia.
Carbon HT'its7s snia.'' They .y'e alone. the s stts%e
i,ssamphihia. to VIII ch all tho recent forms be-
tle earliest, tetraroda. the archipterygial
paired ms having been transformed into the
characteristic Of all. the higher. vertcbrates.
The Uranium is %roofel, over by derma! bones,
supra-orbitals and superintendent are always
present. The steGocephali. taken as a whole.
lead mne1crpiisiy to- the Proreptilia. which
and am m S]FiG9t 94Iar [ f6sHi,s trGm tbs rf e e rHGp i,t these
many undouhtcd fossil anrphibia were mis-
taken for rept! les. rs Indicated by the ~freqGent
nearest living, representative Of these extinct
proreptlIla is tho New Zea,and reptile, hattcria.
known from the Unper Trias. while others
have been discovered in the Permian. Any
nearest e The main stem Of our ancestors.
The most important CHARACTERISTICS OF the rep
lila which mark 4 stage or Level higher than
precision OF the EilIs, although, during the cm-
bryonic development. he gill clefts still appear
In nil reptiles, birds and mammals: sec
ondly. the drvelOpment OF! all nmnion
and an aIlantois, both for the embryonic
these animals are comprised under. the name
Of amnlota: thirdly. the articulation Of the
ycloped condyles. either. single or double:
fourthly. the formation OF contra. oil, bodies, Of
t trfsi ysfTts snnGr manly "n dSn Cy 4 tn central B Esl wc OF the the
proreptilia and the mammalla. which latter &C.
curin the Ipn"r Triassic epoch, must be inter
still sogenernlL ed that heirdescendants could
branch of either into the, %reptilia proper Of into
tho mammalia. The changes concerned are
chiefly thoae OF the brain and the heart, of the
skeleton. the skull and the peIvis and, of tho
| ton Of a hairy covering, Many such creatures
existed in the Triassic epoch. namely. the thero-
act rrinstiics Which, oSthPerrwaiise Occur in the nam
malia only that these creatures have been
term r h5 a%r%,m AL mnns II I has to Ea be smphS,
hitherto discovered tulals all the require
ments which would r entitls It to th,s imGortqnt
us an approximate iden of what the link was
We come now 70 the stage OF the promamma-
liza. or prptotheria. The only surviving mem;
beers OF this group are the famous dnckbill and
These few genera, however. differ so much
from one another ~ various imiortant respects
orig lnHy I n'nch la n T' r' In FRI. = Has cEE@Iln
opinion many fossils from the Upper Triassic
and from the Jnpsslc strata should be re-
pron"unc;d tyEical mammals, because they
the loss Of their nucleus, been modified from
hairy coat and sweat glands. and two occipital
condyles; the ilip;sacral connection is preace-
u]uadra,e bone Of tho rcptilia has ceased to
carry the under Jaw, which now nrticulates di-
TsSuyw Td w I n T }Rnn q inrtom III me skuiL.
shown by the retention OF the following rep
tiIiinn features; CGmplete %corncoid bones and
a T-shnped interclnvirle; a cloaca or common
chamber for the passage of the frees. the uri-
nary md the genital Irodpcts: they are still
rion. aud Is. ih"re'ore, not nourished through
II piacenta. Even the milk glands. which are
absolutely peculiar 10 the mammalia. are still in
a very primitive stage and do not yet produce
milk proper: We observe, finally. that there is
only a temporary shallow marsupium or nurs-
ing bag,
~.
Tho marsupialia, or metatheria. are direct
descendants Of prototheria; but they show
higher development by the reduction Of the
coracoid homes and the lntercIavicles The
original cloaca is divided into Il rectal chamber
anda uro-genital sinus, completely separated.
Mt least in the male; they are viviparous: the
younG are received into a permanent mar