174 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



the lines of descent of Mammalia have originated or 

 been continued through forms of small size. The 

 same is true of all other Vertebrata. 



It is not to be inferred from the reality of the law of 

 "the unspecialized" that each period has been de- 

 pendent on the simplest of preceding forms of life for 

 its population. Definite progress has been made, and 

 highly specialized characters have been gradually de- 

 veloped, and have passed successfully through the 

 vicissitudes of geologic revolutions. But these have 

 not been the most specialized of their respective ages. 

 They have presented a combination of effective struc- 

 ture with plasticity, which has enabled them to adapt 

 themselves to changed conditions. 



In a large number of cases in each geologic age 

 forms have been successful in the struggle for existence 

 through the adoption of some mode of life parasitic on 

 other living beings. Such habits reduce the struggle 

 to a minimum, and the result has been always degen- 

 eracy. In other cases it is to be supposed that ex- 

 tremely favorable conditions of food, with absence of 

 enemies, would have occurred, in which the struggle 

 would have been almost nil. Degeneracy would fol- 

 low this condition also. On the other hand, extreme 

 severity of the struggle cannot have been favorable to 

 propagation and survival, so that here also we have a 

 probable cause of degeneracy. Degeneracy is a fact 

 of evolution, as already remarked, and its character is 

 that of an extreme specialization, which has been, like 

 an overperfection of structure, unfavorable to sur- 

 vival. 



In general, then, it has been the "golden mean" 

 of character which has presented the most favorable 

 condition of survival, in the long run. 



