1 86 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



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 tend- 

 ency to be inherited at an earlier age than those useful 

 only to the adult, and so by unequal acceleration of devel- 

 opment the parallel between ontogeny and phylogeny is 

 broken. It was once thought that the Naiiflius larva of 

 the crustaceans was a mature genus, then it was thought to 

 be a larval representative of the extinct radicle of the Crus- 

 tacea; later still, many morphologists have concluded that 

 the Naufliiis, 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 primitive crustacean, and the 

 two sets of characters are thrown together by unequal accel- 

 eration. Beecher (1895, p. 173, PL IX, figs, i, 2, 4) has 

 shown the same thing in the spiny larvae of Acidasfis 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 characters that 

 would identify them undoubtedly with trilobites they have 

 assumed those most characteristic of their own genera. 

 Jackson (1890, p. 381) has shown that in the larvae of the 

 Pectiiiidce unequal acceleration may associate characters that 

 were not synchronous in race history. F. Bernard ( 1896-97) 

 has recently shown that the prodissoconch of pelecypods is 

 sometimes striated and ribbed, characters that could not have 

 belonged to the primitive pelecypod. 



If unequal acceleration causes confusion in the phylem- 

 bryonic stages, the difficulty is much greater in the larval 

 and adolescent periods, where the shortness of the time of 

 development causes throwing together of characters that 

 were not contemporaneous 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. 



