CRUSTACEA AND MOLLUSCS 251 



the course of the individual development to such an 

 extent that in no single case will it faithfully recapitu- 

 late the ancestral history. It is only rarely that the 

 biogenetic law will help the student to trace the stem- 

 history of an animal. That is not the purport of the 

 law. It serves rather to make more intelligible the 

 action of natural selection, which can only build upon 

 previous structures in the transformation of animals. 

 When we find ancestral traits in the embryonic 

 development of an animal, it is a proof of the theory 

 of descent, but we must not demand such fortunate 

 accidents. The ontogenetic evidence for phylogeny has 

 been so much distorted by natural selection that it is 

 probably the worst we have. It can only rarely serve 

 to illumine the darkness that lies on the past history of 

 organisms, and then generally only in conjunction with 

 the other two sources of evidence, the structure of 

 animals and the geological discoveries. 



We started from the larva-form of the crustacean, the 

 nauplius. In that shape many crustaceans, such as the 

 small hoppers and the branchiopods, leave the egg, and 

 gradually pass into the adult form in the course of their 

 free life. Other crustaceans pass through the nauplius 

 stage in the ovum, and issue from it at a more advanced 

 larva stage, the zoea, which has a larger number of legs 

 and an abdomen. This is the case with most of the 

 higher crustaceans, such as the sea-crabs, which would 

 otherwise have to delay too long in the larva-stage on 

 account of their elaborate articulation and numerous 

 extremities. It is clearly better for the animal if the 

 time is not too protracted until it begins to lay eggs, 



