COMPARATIVE ANATOMY AND MORPHOLOGY 347 



would be admissible. But it is not, and the deviations from the 

 parallelism of ontogenesis and phylogenesis are not recognizable by 

 sure signs. 



The examples in proof of this assertion are not rare. Here is one : 

 when the larva of the echinoderms metamorphoses into the defini- 

 tive form, sometimes it retains the mouth, which is merely dis- 

 placed by passing to the left side; sometimes the mouth closes and 

 a new mouth breaks through on the left side. From the ontogenetic 

 point of view the mouths are not homologous in the two cases; the 

 homologue of the mouth of one of the forms is, in the other, an 

 imperf orate point of the surface of the body. 



Is it necessary, then, to say that in two species of starfish, the 

 adults of which present the two cases given above, Asterina gibbosa 

 and Asterias glacialis, for example, animals as like one another as 

 are the cat and the dog, the two mouths are not homologous, that 

 the representative of the mouth of Asterias glacialis is such or such 

 imperf or ate point on the edge of the disk of Asterina? 1 



It seems from all the evidence that the common ancestor of Asterias 

 and of Asterina had the normal mouth of a starfish, and that this 

 mouth has become the mouth of Asterias, on the one hand, and of 

 Asterina, on the other, so that phylogenetically the mouths of 

 Astenas and Asterina are homologous, although ontogenetically 

 they are not. Nevertheless, some morphologists, ready for anything, 

 do not fear to reject this common-sense conclusion, and to declare 

 that the mouth of Asterias and that of Asterina are not homo- 

 logous. 



But on the other hand many morphologists reject that conclu- 

 sion, and in other cases, otherwise entirely similar, all agree to take 

 the reverse of that fashion of reasoning. 



Among the insects, the embryo shows at the beginning of its 

 development an imagination which appears exactly like that which 

 in the majority of other animals (including the other arthropods) 

 gives rise to the primitive gastric cavity. If it became that cavity, 

 we would not fail to homologize it with the archenteron, but since 

 it develops into something entirely different, the mesoderm, we pass 

 in silence over this homology, thus making an exception to our 

 established principles. 



Why do we make such an exception to our principles? Because, 

 in spite of our professions of faith, we do not take as a criterion 

 ontogenesis alone as a copy of phylogenesis. When the ontogenetic 

 homology is too strongly opposed to a certain intimate sense of 

 homology which we have within us, we lay it aside, passing it over 



1 In the same way we refuse to recognize as similar the general body-cavities 

 of two animals, in spite of resemblances which render them almost identical, when 

 one arises as an enterocoel and the other as a schizocoel. 



