136 HYPOTHESIS OF THE DEVELOPMENT OF 



and among insects the Ametabolous, first appeared upon our 

 earth." So much for the Invertebrate sub-kingdom. 



When we proceed higher, we find the examples become 

 even more remarkable. Take the White-fish, one of the most 

 highly organized of its class, which, in mature life, has a bony 

 skeleton, an equally forked tail (Fig. 25), and a mouth in the 

 front of its head. When it first comes forth from the egg, the 

 vertebrae are cartilaginous, the caudal fin unequally forked, 

 and the mouth transverse and underneath its head, resembling 

 generally the mature cartilaginous fish. Now, as we have 

 seen, the cartilaginous fishes existed long before there were 

 any possessing osseous skeletons. Take the Frog : it is at its 

 birth a tadpole, with external branchiae and other organs 

 fitting it to live in the water. It is, in short, essentially a 

 fish. We should therefore expect that fishes should have 

 existed before frogs, which the geological record fully testifies. 

 Moreover, the oldest batrachia, as archegosaurus, are esta- 

 blished by Owen as of the same character as those which we 

 find retaining through life ichthyic peculiarities. 1 



Such are a few of the more prominent facts tending to 

 establish a parity or identity of plan between the succession 

 of animals on the earth, and the stages of embryonic develop- 

 ment in those which have last come upon the scene. This 

 parity becomes the more striking, from those very peculiarities 

 which have been felt as most puzzling in both. Some steps 

 in the march of fossils have been thought to be so inconsistent 

 with the idea of a general progress of life, as to cause many to 

 repudiate that idea entirely. A distinct conception of embry- 

 onic development ought to reconcile them to it. We must 

 take into view that, besides the vertical movement of the 

 embryo through grades of being, there is a kind of lateral 

 advance from the more general to the more special. The 

 characters of the foetal animal are at one time more compre- 

 hensive than they afterwards become. We should, therefore, 

 in the assumption of a perfect parity in the two sets of phe- 

 nomena, expect that the first fishes, while the lowest, should 

 also partake of the characters of other vertebrate, and even of. 

 certain invertebrate orders, which appears to be precisely the 

 case with those of the Old Red Sandstone. The Cephalaspids, 

 while, 'in their external skeleton, leaning to the Crustacea, were, 

 as a distinguished physiologist has remarked, " shaped like the 



1 Art. Palaeontology, Encyc. Brit. 



