THE BIOGENETIC LAW 4 1 9. 



acceleration would account for a large part of the differ- 

 entiation observed in successive geologic generations. 



Retardation. — Another factor that makes it difficult to cor- 

 relate ontogeny and phylogeny is retardation of development. 

 Cope first recognized the principle, but in his writings confused 

 it with unequal acceleration, and since his reasoning was purely 

 theoretical the idea has never gained much foothold in biologic 

 philosophy. Cope's 1 statement of the theory is as follows: 

 "The acceleration in the assumption of a character, progressing 

 more rapidly than the same in another character, must soon 

 produce, in a type whose stages were once the exact parallel of 

 a permanent lower form, the condition of inexact parallelism. 

 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 evolution during the long ages 

 of geologic time. Each type has, however, its day of suprem- 

 acy and perfection of organism, and a retrogression in these 

 respects has succeeded. This has, no doubt, followed a law the 

 reverse of acceleration, which has been called retardation. By 

 the increasing slowness of the growth of the individuals of a 

 genus, and later assumption of the characters of the latter, they 

 would be successively lost." This statement of Cope might 

 apply equally well to unequal acceleration of characters, but in 

 another part of this same work he gives a clearer statement : 

 "Where characters which appear latest in embryonic history are 

 lost, we have simple retardation, that is, the animal in successive 

 generations fails to grow up to the highest point of completion,, 

 falling further and further back, thus presenting an increasingly 

 slower growth in the special direction in question." 2 



These remarks of Cope were based on abstract reasoning,, 

 but it is possible to bring up some striking cases in support of 

 the theory, notably among the brachiopods. Fischer and 

 Oehlert 3 have shown that while brachiopods go through many 

 metamorphoses in individual evolution, and while each species 



1 Origin of the Fittest, p. 142. 2 0p. cit., p. 13. 



3 Brachiopodes, Mission Scientif du Cap Horn, p. 50-60. 



