ORGANIC EVOLUTION — THE FACTORS 99 



of the Cessation of Use, but only by supposing that the 

 accumulation of inborn variations has resulted in an 

 overgi-owth of protective skin. 



This latter, however, is a case of evolution, not of re- 

 trogression, and is therefore not due to " what has been 

 called in questionable logic," reversed selection — " the 

 selection which effects not increase of an organ but 

 decrease of it" (p. 13). 



Reversed Selection is supposed by some biologists 

 to have operated in another way as a cause of retro- 

 gression, viz. by effecting economy of nutrition. They 

 hold that when in any species an organ becomes useless, 

 those individuals that vary in such a maimer that in 

 them supplies of nutrition are diverted from the useless 

 organ to useful organs are at an advantage over those 

 individuals which do not so vary, and that this advan- 

 tage, due to economy of nutrition, is great enough to be 

 an important factor in survival, and that to it therefore 

 is due in whole or part the retrogression of a useless 

 organ. 



In championing the opinion that retrogression is due 

 to disuse, Mr. Spencer directs his whole powers of 

 attack against this last theory, quite ignoring the other 

 theories I have discussed, which attribute retrogression 



(1) to Cessation of Selection as regards useless organs, or 



(2) to Reversed Selection operating against worse than 

 useless organs, or (3) to the combined action of both Ces- 

 sation of Selection and Reversed Selection. So far as 

 his arguments go, I thitik it must be admitted that they 

 are conclusive. 



He says, " Suppose that in a new habitat the 

 kangaroo had no enemies; and suppose that, conse^ 

 quently, quickness of hearing not being called for, large 

 ears gave no greater advantage, than small ones. 

 Would an individual with smaller ears survive and 

 propagate better than other individuals in consequence 



