GEOGRAPHY AND EVOLUTION: 201 



The plants and animals existing at any time in any locality tend con- 

 stantly to ditiuse themselves around that local centre, this tendency 

 being controlled by the conditions of climate, etc., of the suri'ounding 

 area, so that under certain unfavorable conditions diffusion ceases. 



The possibilities of life are further seen to be everywhere directly 

 influenced by all external conditions, such as those of climate, includ- 

 ing temperature, humidity, and wind ; of the length of the seasons 

 and days and nights ; of the character of the surface, whether it be 

 land or water ; and whether it be covered by vegetation or otherwise ; 

 of the nature of the soil ; of the presence of other living creatures, 

 and many more. The abundance of forms of life in difterent areas 

 (as distinguished from number of individuals) is also found to vary 

 greatly, and to be related to the accessibility of such areas to immi- 

 gration from without ; to the existence, within or near the areas, of 

 localities offering considerable variations of the conditions that chiefly 

 afiect life ; and to the local climate and conditions being compatible 

 with such immigjration. 



For the exjjlanation of these and other phenomena of organization 

 and distribution, the only direct evidence that observation can supply 

 is that derived from the mode of propagation of creatures now liv- 

 ing ; and no other mode is known than that which takes place by 

 ordinary generation, through descent from parent to offspring. 



It was left for the genius of Darwin to point out how the course 

 of Nature, as it now acts in the reproduction of living creatures, is 

 sufficient for the interpretation of what had previously been incom- 

 prehensible in these matters. He showed how propagation by descent 

 operates subject to the occurrence of certain small variations in the 

 offspring, and that the preservation of some of these varieties to the 

 exclusion of others follows as a necessary consequence when the exter- 

 nal conditions are more suitable to the preserved forms than to those 

 lost. The operation of these causes he called Natural Selection. Pro- 

 longed over a great extent of time, it supplies the long-sought key to 

 the complex system of forms either now living on the earth, or the 

 remains of which are found in the fossil state, and explains the rela- 

 tions among them, and the manner in which their distribution has 

 taken place in time and space. 



Thus we are brought to the conclusion that the directing forces 

 which have been efficient in developing the existing forms of life from 

 those which went before them are those same successive external con- 

 ditions including both the forms of land and sea., and the character 

 of the climate, which have already been shown to arise from the 

 gradual riiodificaiion of the material fabric of the globe as it slowly 

 attained to its present state. In each succeeding epoch, and in each 

 separate locality, the forms preserved and handed on to the future 

 were determined by the general conditions of surface at the time and 

 place; and the aggi-egate of successive sets of conditions over the 



