THE BIOGENETIC LAW 417 



then not yet ended; for the characters do not necessarily appear 

 in the ontogeny of the descendant in the same association in 

 which they occurred in the ancestor. A character useful to the 

 immature form will have a tendency to be inherited at an earlier 

 age than those useful only to the adult, and so by unequal 

 .acceleration of development the parallel between ontogeny and 

 phylogeny is broken. It was once thought that the Nauplius 

 larva of the crustaceans was a mature genus, then it was thought 

 to be a larval representative of the extinct radicle of the Crusta- 

 cea ; later still, many morphologists have concluded that the Nau- 

 plius, while it bears many crustacean characters, still retains too 

 many annelid characters to represent the radicle of the group ; it 

 is a typical crustacean larva, but not a representative of the primi- 

 tive crustacean, and the two sets of characters are thrown together 

 by unequal acceleration. Beecher has shown the same thing in 

 the spiny larvae of Acidaspis and Arges, where in the protaspis 

 of these genera the spines characteristic of the adults appear, 

 contrary to usage among the trilobites, in which larval stages are 

 usually smooth. Thus before these animals have assumed char- 

 acters that would identify them undoubtedly with trilobites they 

 have assumed those most characteristic of their own genera. 

 Jackson has shown that in the larva?, of the Pectinidce unequal 

 acceleration may associate characters that were not synchronous 

 in race history. F. Bernard has recently shown that the pro- 

 dissoconch of pelecypods is sometimes striated and ribbed, 

 characters that could not have belonged to the primitive pelecy- 

 pod. 



If unequal acceleration causes confusion in the phylembry- 

 onic stages, the difficulty is much greater in the larval and ado- 

 lescent periods, where the shortness of the time of development 

 causes throwing together of characters that were not contempo- 

 raneous in the ancestors, and where the small size and general 

 habits prevent differentiation of organs that in the correlative 

 adult forms were highly developed, thus obscuring and even 

 destroying the exactness of the parallelism. Two species of 

 Placenticeras, of which the ontogeny has been recently studied 



