The Recapitulation Theory in Biology 25 



these added ideas was that of unequal acceleration, concerning 

 which Cope makes this statement: 



"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 paral- 

 lelism. As all the more comprehensive groups present this 

 relation to each other, we are compelled to believe that accelera- 

 tion has been the principle of their successive evolution during 

 the long ages of geological time." 36 



Two other extracts from another writer will show more fully 

 the meaning of this principle: 



"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"; 



and this unequal acceleration is especially to be expected 



"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. . . "" 



The other idea added to the law of acceleration was named by 

 Cope " retardation.' ' 



"Where characters which appear latest in embryonic history 

 are lost, we have simple retardation, that is, the animal in suc- 

 cessive generations fails to grow up to the highest point of com- 

 pletion, falling further and further back, thus presenting an in- 

 creasingly slower growth in the special direction in question. ' m 



This gives rise to retrogressive series in evolving forms in 

 contrast with progressive series resulting from the effect of 

 acceleration. 



The bearing of these principles of acceleration, unequal ac- 

 celeration, and retardation upon the theory of recapitulation 

 has now to be indicated. If acceleration should operate uni- 

 formly upon lines of descent without obstruction, plainly the- 



" Origin of the Fittest, p. 142. 



•' J. P. Smith, Journ. of Geology, Vol. 8, 1900, p. 417. 



" Origin of the Fittest, p. 13. 



