38 ASPECTS OF BIOLOGY AND METHOD OF BIOLOGICAL STUDY. 
Through the hypothesis of evolution, paleontology and embryology 
are thus brought into mutual bearing on one another. 
Let us take an example in which these two principles seem to be 
illustrated. In rocks of the Silurian age there exist in great pro- 
fusion the remarkable fossils known as graptolites. These consist 
of a series of little cups or cells arranged along the sides of a com- 
mon tube, and the whole fossil presents so close a resemblance to 
one of the Sertularian hydroids, which inhabit the waters of our 
present seas, as to justify the suspicion that the graptolites consti- 
tute an ancient and long since extinct group of the Hydroida. It 
is not, however, with the proper cells or hydrothece of the Sertu- 
larians that the cells of the graptolite most closely agree, but rather 
with the little receptacles which in certain Sertularinæ belonging to 
the family of the Plumularida we find associated with the hydro- 
thecæ, and which are known as ‘*Nematophores.” A comparison 
of structure then shows that the graptolites may with considerable 
probability be regarded as representing a Plumularia in which the 
hydrothecs had never been developed and in which their place had 
been taken by the nematophores. 
Now it can be shown that the nematophores of the living Plu- 
mularida are filled with masses of protoplasm: which have the — 
power of throwing out pseudopodia, or long processes of their sub- — 
stance, and that they thus resemble the Rhizopoda, whose soft parts 
consist entirely of a similar protoplasm and which stand among the 
Protozoa, or lowest group of the animal kingdom. If we suppose ~ 
the hydrotheca suppressed in a plumularian, we should thus nearly a 
convert it into a colony of Rhizopoda, from which it would differ — 
only in the somewhat higher morphological differentiation of its — 
cænosarc or common living bond, by which the individuals of the — 
colony are organically connected. And, under this view, just such 
a colony would a graptolite be, waiting only for the development — | 
of hydrotheca to raise it into the condition of a plumularian. 
Bringing now the evolution hypothesis to bear upon the ques- 
tion, t would follow that the graptolite may be viewed as an an- — | 
cestral form of the Sertularian hydroids, a form having the most 
intimate relations with the Rhizopoda; that.hydranths and hydro- — 
thecze became developed in its descendants ; and that the rhizo- 
podal graptolite became thus converted, in'the lapse of ages, into 
the hydroidal Sertularian. — 
This hypothesis would be strengthened if we found it agreeing 
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