192 Changing Plant and Animal Nature 



to moment, also, the individual responds to what is going on 

 without, and its various parts coordinate and compensate 

 their processes in relation to one another. Every random 

 grouping of living things is constantly changing. Its con- 

 stituent plants and animals undergo their respective fluctua- 

 tions of activities from moment to moment. The changing 

 season imposes conditions of growth and decline. The plants 

 and animals devour one another and exchange materials and 

 services. There is the further alteration of the group by 

 natural deaths, by emigration and by immigration. If we 

 consider a group like a species, we again find variability a 

 constant feature. The composition of the living population 

 is constantly changing for the reasons suggested as applying 

 to the living individual and to the random group. In ad- 

 dition, the pressure of population tends always to spread the 

 individuals in all directions. New conditions, new situa- 

 tions have to be met. Since no two individuals are alike, 

 and since no two situations are alike, conditions are met with 

 all degrees of effectiveness or failure. The composition of 

 the species is thus flowing now in the direction of the swift, 

 now in the direction of the tough, now in the direction of 

 the sensitive, and so on. 



The evidence does not show that the selection of the 

 ordinary " struggle for existence " steadily changes the make- 

 up of a species in a consistent direction. It does show, how- 

 ever, that in addition to the adjustability of the individual 

 to the short-time changes, there is an adaptability of the 

 species to long-time changes. It shows, moreover, that cor- 

 responding to the plasticity of the individual is the varia- 

 bility of the species, and that certain types of variables are 

 preserved in succeeding generations. In other words, al- 

 though variability is not a proof of evolution, in the historical 

 sense, it is an essential condition of the transformation of 

 species. And it is seen to be ever present in a form that 

 makes evolution or the transformation of species not only 

 possible, but the most probable outcome of the constant 

 change which is life. 



