146 Changing Plant and Animal Nature 



selves independently from generation to generation. It is 

 not sufficient for our purpose that the individual merely 

 deviates from the pattern of his species — mutilating and 

 destructive effects of disease and injury yield deviations 

 enough. It is not sufficient that individuals be born that 

 diverge from the parental pattern — monstrosities are com- 

 mon enough. It is necessary for our purpose to find devia- 

 tions from the pattern set by the species and divergences 

 from parental models, that are at the same time capable of 

 living and of perpetuating themselves. 



To be able to recognize evolution taking place, we must 

 consider the facts of adaptation as well as the facts of varia- 

 tion and of heredity. The inhabited parts of the world show 

 great extremes of physical and chemical conditions — of 

 degrees of moisture or dryness, of heat or cold, of light and 

 shadow and darkness, of chemical composition, of concen- 

 tration and of proportions of various substances. In all of 

 these varied conditions, we find living things, animals as 

 well as plants, and many living things of which we cannot 

 say definitely that they are either plants or animals. In the 

 various dwelling places or habitats, these living forms differ 

 from one another. The plants and animals of the water are un- 

 like the plants and animals of the land. The inhabitants of 

 the desert are unlike those of the plain or of the swamp. 

 The plants and animals of fresh water differ from those of 

 the salt sea, and those afloat in the water at large differ from 

 those along shore or in the depths. 



In every situation, however, we observe that the living 

 things are somehow fit to live, they show adaptation. Their 

 structural and functional peculiarities are suited to the 

 habitat. A part of the problem of the origin of species is to 

 find out just how living things come to fit so remarkably 

 well their respective abodes. 



Fitness of the Environment 



We are disposed to take for granted the physical world 

 as something fixed and final, and to think of fitness as a one- 



