150 THE PROBLEMS OF EVOLUTION 



ment and observation more accurately to the con- 

 ditions prevailing in nature. 



Evolution is a process of change, therefore we 

 must start with a homogeneous stock if we are to 

 explain it satisfactorily. We may not even admit 

 the normal hereditary complex of a single species 

 like those of the present as a point of departure if 

 we are to consider the full meaning of evolution, 

 for the diversity of characters within the species 

 is as much a product of evolutionary processes as 

 the variety of species now inhabiting the world. 

 A few factors only must provide our materials. As 

 a natural property of living things, our hypotheti- 

 cal organisms will reproduce, even to such an ex- 

 tent that their initial home will be overcrowded. 

 They will migrate, or be gradually dispersed by 

 other means. Finally the conditions under which 

 they live will fluctuate through forces over which 

 living things have no control and of whose ulti- 

 mate causes man himself knows nothing as yet. All 

 of the fluctuations of environment which we have 

 considered may impinge upon our homogeneous 

 stock as a result of these fundamental causes. They 

 may be subjected to variations of temperature, of 

 moisture, of food, of light, of cosmic rays, of me- 

 chanical contacts, of acidity, and of many other 

 things. 



At the outset natural rays acting on the similar 

 materials of many individuals may bring about 



