250 EVOLUTION AND GENETICS 



must be adapted to both extremes. The same quahty is expressed 

 in short summers and long winters at high altitudes even in lower 

 latitudes. Snow may be expected throughout the year at 15,000 

 feet in latitudes where it never falls at sea level. 



A geological change such as the elevation of continents would 

 change the climate of a region completely. If local, in the form 

 of elevation of a mountain range, it would have a similar effect 

 on the region involved, but would exert various influences on 

 other parts of the same land mass. Its modification of precipita- 

 tion might be extreme. Drainage might be affected in such a 

 way as to change the distribution of fresh waters and the char- 

 acter of the streams. These changes could not fail to modify the 

 environment of many organisms. 



Climatic Factors and the Food Supply. As a result of geo- 

 logical changes and their indirect influences, the flora of a continent 

 is likely to be modified. One of the most conspicuous indications 

 of the climate of past geological periods is, in fact, the nature 

 of the plant remains. This is a natural outcome of the intimate 

 association of plants with the physical environment. Change of 

 climatic factors to them means direct modification of the food 

 supply, and their response, in turn, means modification of the 

 food supply of all other organisms, directly or indirectly. Through 

 the web of life a change may be transmitted to unsuspected lengths. 



Food supply is one of the most important contributions of 

 the environment to individual life. Its modification is therefore 

 a matter of deep concern, perhaps more vital than the modification 

 of other factors, but it is easily seen to be intimately linked with 

 climatic factors on one hand, as a causative factor in evolution. 



Rate of Reproduction. No less intimately associated with 

 modification of the food supply is the factor of overproduction, 

 although in this case it is not the actual supply that is modified, 

 but its availability to individuals. The principle of overproduction 

 was emphasized by Malthus in his work on population, which 

 furnished the inspiration for Darwin's and Wallace's theories 

 of natural selection. In man it seems less conspicuous than in 

 animals, although man was Malthus' subject and the same process 

 was later recognized in animals. It is obvious, however, that all 

 organisms produce more individuals than can survive, and that 

 only constant destruction prevents overpopulation. Interruption 

 of the natural balance by transportation of animals to foreign 



