134 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



portion to their removal in time and position from the proto- 

 conch, or last embryonic stage." ^ 



The life history of the ammonites is the best example of 

 the law of tachygenesis ; these branched off from the nau- 

 tiloids near the beginning of Devonian time, continued 

 increasing, diverging, became highly specialized and accel- 

 erated until their final extinction at the end of Cretaceous 

 time. Each ammonite goes through a larval history that is 

 long and varied in direct proportion to the length of time 

 from its period back to the Lower Devonian. Thus the 

 Nautilinidce, the first of the new stock, have a compara- 

 tively simple ontogeny, there being no great changes from 

 the larval up to the adult stages. The higher Devonian and 

 Carboniferous forms go through several generic changes 

 before they become adults, and Mesozoic genera have still 

 longer larval and adolescent periods; that is, longer in the 

 sense of more complicated. 



A distinct addition to this principle has been made in 

 Cope's idea of retardation, by which is explained the sepa- 

 ration in the ontogeny of the descendant of characters that 

 occurred simultaneously in the ancestor. Cope says : " The 

 acceleration in the assumption of a character, progressing 

 more rapidly than the same in another character, must pro- 

 duce, in a type whose stages were once the exact parallel 

 of a permanent lower form, the condition of inexact ^aral- 

 lelisni. As all the more comprehensive groups present this 

 relation to each other, we are compelled to believe that 

 acceleration has been the principle of their successive evo- 

 lution during the long ages of geologic time. Each type 

 has, however, its day of supremacy and perfection of 

 organism, and a retrogression in these respects has suc- 

 ceeded. This has no doubt followed a law the reverse of 

 acceleration, which has been called retardation. By the 

 increasing slowness in the growth of the individuals of a 

 genus and later assumption of the characters of the latter, 

 they would be successively lost."^ 



1 Proc. Amer. Phil. Soc, Vol. XXXII, No. 143, p. 405. 



2 " Origin of the Fittest," p. 142. 



