328 CONCLUSION 



result of something else, and we are far from understanding 

 how it is produced in the first instance. 



Embryogeny does not reproduce only ancestral characters. 

 Free embryos, in the course of their development, very 

 frequently modify their mode of life ; they return to the con- 

 ditions in which these characters were achieved, thus allowing 

 us to discover their causes, so that we are able to emphasize 

 the importance of attitudinal changes in the realization of 

 organic types, whose structure did not appear at first sight to 

 be referable to any causes within the scope of our observation. 

 This conviction led Cuvier to postulate his four immutable 

 structural types, of which, however, only one, the Vertebrata, 

 was clearly delimited by constant and precise characters. For 

 Cuvier's four " embranchements " we have to-day substituted 

 nine : Protozoa, Porifera, Ccelenterata, Chitinophora (Arthropoda 

 and Nemathelminthes) , Vermes, Echinodermata, Mollusca, 

 Vertebrata, and, related to the last-named by a process of 

 degeneration due to the fixation of their embryos to objects 

 below the surface of the sea, Tunicata. But for each one of these 

 phyla a clear explanation has been given of the characters 

 that distinguish it. It is extremely improbable that deep-sea 

 research will provide us with any new phyla, for it would 

 seem as though those we already know correspond to all the 

 types that are rationally possible. But only four of these 

 phyla have dowered the freshwater or the solid land with a 

 numerous posterity ; to wit, the Chitinophora, whose essential 

 types are represented by the Arachnida and Insecta ; the 

 Vermes, Mollusca, and Vertebrata. We have already seen how 

 the hermaphroditism of the Vermes and the lacustrine and land 

 Molluscs has raised the question of conditions which determine 

 the production of this or that sex. 



It would seem that the males of the lower forms show little 

 aptitude for development, and are relatively weak, and that 

 those of the higher forms show a disposition to squander their 

 food reserves in the production of useless ornaments such as the 

 brilliant plumes of male birds, the decorations of numerous 

 male insects, the mane of the lion, the beard of man, etc., or 

 in organs of defence and attack such as the horns of the various 

 male ruminants, the tusks of elephants, or the enormous 

 mandibles of the stag-beetle. The females, on the contrary, 

 at least in the Animal Kingdom, generally appear to sacrifice 



