ANIMAL AFFINITIES 291 



malian embryo that cannot possibly have belonged to animals 

 in the mammalian ancestry — the amnion, for instance, which is 

 a membrane completely enclosing the embryo. So also the 

 salmon larva has a large yolk sac attached to its abdomen and we 

 cannot easily imagine an adult fish that had such a structure. The 

 foetal chick has a special structure on its beak which is adapted 

 to break open the egg-shell when the chick comes to hatch. No 

 bird that lived in the past could have had such a structure in its 

 adult state. And so on. 



Further, eggs, embryos and larvae of all animals, at all stages 

 are specifically characterized. A plaice egg, embryo and larvae, 

 are all recognizably plaice and cannot be confused with the egg, 

 embryo or larva of any other fish. A rabbit does not pass through 

 a piscine stage — at every stage of development it is demonstrably 

 a rabbit — and no other animal species. 



Nevertheless, the tectonics of a development is a very conserva- 

 tive process. In the course of animal evolution this tectonic 

 process changed so that its result was some new animal form — 

 as we have seen above — and the changes occurred in such ways 

 that records have been preserved. Although lungs, and the bones 

 of the middle ear, and cerebral hemispheres are present in a rabbit 

 and not in the lower fishes yet we see, from the rabbit ontogeny, 

 that the anlagen of the lungs are the same structures that are the 

 anlagen of the swim-bladder in the fish ; the anlagen of some of 

 the auditory ossicles of the rabbit are the same things as the 

 anlagen of some parts of the fish branchial skeleton and the anlagen 

 of the rabbit cerebral hemispheres are the same things as the 

 anlagen of the piscine pallium. By " the same things " is meant, 

 of course, that the pairs of anlagen develop in the same ways from 

 the pre-existing embryonic parts — that is, they are homologous 

 structures. This means that there are resemblances in the 

 ontogenies of animals : the developments of diff"erent mammals 

 — say a whale and a rabbit — are more similar than are the develop- 

 ments of a mammal and a fish and thus whale and rabbit exhibit 

 closer affinities than whale and fish. And the developments of 

 a mammal and a fish are far more similar than are the develop- 

 ments of a fish and a crustacean. Up to some phase, the develop- 

 mental tectonics of many animals may be very similar and this 

 is indicative of their affinities. The fact that we can discern 

 piscine, developmental tectonics in the embryo of a mammal, but 



