EVOLUTION OF THE COLORS OF BIRDS. 73 



found a mate the variability of the species would not 

 increase, does not, of course, apply to the law of sexual 

 intensification, which has, it seems, been placed upon a 

 firm basis through the researches of Messrs. Geddes and 

 Thomson. 



Cope first enunciated the law of acceleration and re- 

 tardation. Two passages may be cited from his writings 

 explanatory of this law: " I believe that this is the 

 simplest mode of stating and explaining the law of varia- 

 tion; that some forms acc{uire something which their 

 parents did not possess; and that those which acquire 

 something additional have to pass through more numer- 

 ous stages tlian their ancestors; and those which lose 

 something pass through fewer stages than their ances- 

 tors; and these processes are expressed by the terms ' ac- 

 celeration " and 'retardation.""^" On another occasion 

 he had expressed the law thus: " It was also shown that, 

 if the embryonic form were the parent, the advanced de- 

 scendant was produced by an increased rate of growth, 

 which phenomenon was called acceleration; but that if 

 the embryonic type were the offspring, then its failure 

 to attain to the condition of the parent is due to the 

 supervention of a slower rate of growth ; to this phenom- 

 enon the term retardation was applied."! 



With regard to the cause of acceleration Prof. Cope 

 first made the following suggestion:! " The successively 

 higher degree of oxidization of the blood in the organs 

 designed for that function, whether performing it in 

 water or air, would certainly accelerate the performance 

 of all the vital functions, and among others that of 

 growth. Thus it may be that acceleration can be ac- 

 counted for, and the process of the development of the 



*Origm of the Fittest, p. 297. 

 tl. c, p. 125. 

 tL c, p. 14.3-144. 



