TRANSFORMATION AND MONOTYPIC EVOLUTION. 187 



2. Eight Forms of Monotypic Evolution 



Let us now consider how this initial segregation, which is always 

 present in the case of a small colony, is enhanced and intensified by the 

 cooperation of other principles, and how forms segregated through 

 possessing different characters in some one respect come to diverge in 

 other respects. For example, when differences of color become the 

 occasion for sexual and social segregation, how does this open the way 

 for divergent transformation in habits of feeding and in a thousand 

 other respects ? The principles cooperating with independent genera- 

 tion in producing this enhanced divergence are all causes of simple 

 transformation, or monotypic evolution when there is free intergen- 

 eration. Divergent breeds of domestic animals have always been 

 produced when the different sections of a species in the care of different 

 races of men have been prevented from interbreeding, thus securing 

 their independent transformation during the process of domestication. 

 So in nature, when any form of independent generation has been 

 established, any cause of transformation that may afterwards arise 

 will always produce more or less divergent evolution, and never that 

 which is in every respect parallel. But we must defer the discussion 

 of this subject till we have enumerated the more manifest of the prin- 

 ciples of monotypic evolution : 



(1) Assimilational transformation, or modification due to deficiency 

 with economy, or redundance with profusion, of growth, resulting 

 from different degrees of assimilative power. ' ' Economy of growth " 

 is a term already in use, but a term is needed that shall include both 

 this and its opposite. 



(2) Stimulational transformation, or modification produced by 

 changed motions in the fluids of the organism responsive to changed 

 influences in the environment. Under this principle we may place 

 the direct influences of light, heat, electricity, the dampness of the air 

 or the saltness of the water in which the organism is bathed, the qual- 

 ity of the food, and all stimulation from physical and chemical causes, 

 exclusive of those resulting in muscular activity or the movement of 

 the organs. 



(3) Suetudinal transformation, or modification due to the effects of 

 use, disuse, and habitual effort in producing motions, and in resisting 

 the strain of gravity and other forces tending to produce motion. Sue- 

 iude is not found in the dictionary, but I venture to use it as including 

 assuetude, which is being accustomed to, being practiced in, habitual 

 use ; and desuetude, which is disuse, discontinuance of practice. 



