b STUDIES IN EVOLUTION 



present and the causes always operative. Throughout life 

 the individual changes, and in addition varies from all other 

 individuals. The family also changes with time, and like- 

 wise differs from other families. Variation is everywhere 

 present. Moreover, it is generally accepted, and is so taken 

 here, that in its results this variation is not haphazard, 

 but is normally in accordance with certain demands or in 

 harmony with certain surroundings. Whether an organism 

 itself tends to vary in all directions, or is chiefly subject to 

 modifications from external forces, does not alter the preced- 

 ing statement. 



Cope 11 has considered variation as either physico-chemical 

 (molecular) or mechanical (molar). The influence of the 

 first is known as physiogenesis and of the second as kineto- 

 genesis. In the animal kingdom the potency of kinetogenesis 

 is greater as an efficient cause of evolution; while in the 

 vegetable kingdom physiogenesis is apparently of more 

 importance. 



The tendency of variation is always in the direction of the 

 establishment of an equilibrium between the organism and its 

 environment. However, the laws of the development of the 

 earth preclude the possibility of a constant environment, and 

 therefore a perfect, permanent, and uniform equilibrium 

 between life and surroundings is unattainable. 



The manner of variation is clearly defined as progressive 

 and regressive. Progressive variation is one of the essential 

 factors of evolution, while regressive variation is towards 

 dissolution. Since the main history of life is told through 

 processes of the former, progressive variation is far greater 

 in importance ; while, in general, regressive variation can be 

 applied only to late periods in the history of groups or forms 

 now in their decadence, or to others which in past times have 

 suffered decline and extinction. 



The summary of the operation of the law of multiplication 

 of effects, as given by Herbert Spencer, ^^ may well be stated 

 here, as it emphasizes one of the principles through which 

 spines have originated. 



