74 GENERAL EVOLUTION. 



6. The Extent of Parallelism. 



Prof, de Serres and others have stated it as their belief that 

 the lower '* branches" of the animal kingdom are identical with 

 the undeveloped forms of the higher ; i. e., that the mollusk and 

 articulate are not merely parallel with, but the same as the lower 

 conditions of, the vertebrate. The works of various embryologists, 

 as von Baer and Lereboullet, have shown this statement to be 

 erroneous, "and founded on false and deceptive appearances." 

 The embryos of the four great branches of the animal kingdom 

 appear to be distinct in essential characters from their first 

 appearance. But Lereboullet, who, in his prize essay, has 

 compared with care the development of the trout, pike, and 

 perch of the Teleosts, with that of a Lacerta among reptiles, has 

 failed to point out characters by which the embryos of the two 

 vertebrate classes essentially differ, for a considerable period. It 

 is true that, as each and all of the species belong to widely differ- 

 ent generic series, parallelism is of the kind to be called iiiexact 

 or remote. But enough is known of embryology and paleontol- 

 ogy to render it extremely probable that the historic predecessors, 

 of the types whose embryology Lereboullet studied, formed a se- 

 ries of parallels of the kind termed in this essay exact. 



Lereboullet states that a certain difference exists between the 

 eggs of the fishes and those of the Lacerta. This is for us merely 

 stating that the parents of the embryos differ, a fact which no one 

 will contest. The same may be said of the elevated or depressed 

 character of the surface of the vitellus on which the embryo re- 

 poses. 



Secondly, after the appearance of the embryo the Lacerta is 

 furnished with the amnios and allantois ; the Teleost not. This 

 is certainly neither a generic, ordinal, nor class character of the 

 adult, for it is but temporary ; therefore, in generic, ordinal, and 

 class characters the embryos of the Teleost and Eeptile are still 

 identical. It is a physiological character, and not morphological, 

 and therefore far the less likely to be a permanent one, even in 

 embryos, under changed circumstances. The female of one of 

 the species of Trachycei^halus inverts the skin of the back at one 

 season of the year to receive her eggs, because she can not lay 

 them in the water ; the other species of the genus do not. The 

 next genus in direct morphological line possesses a single species 

 whose female does the same for the same reason ; but the rela- 



