NATURAL SELECTION 103 



ever, occurs only in exceptional cases. If, as we are bound to 

 do, we recognise Natural Selection as an integral part of the 

 system at work around us, we must conclude that advance 

 towards higher and more complex forms of life is the pre- 

 dominant tendency. How else can we account for the fact that 

 such forms are found in all the great land areas of the world ? 



INTER-ACTION OF SPECIES 



All species must conform to their environment. If the environ- 

 ment remains unaltered, evolution ceases. For Natural Selection 

 can only give to a species a bare superiority to the conditions 

 under which it lives. The great amount of elimination that goes 

 on even when the conditions remain unaltered, tends, as I have 

 shown in the last section, to give fixity to existing characters. 

 Further evolution can take place only if an environment that is 

 in some way different offers itself, whether the difference con- 

 sists in the introduction of some novel feature or merely some 

 intensification, some raising of the standard of life in the existing 

 milieu. 



This being the case, physical conditions are a comparatively Compara- 

 unimportant factor. They remain unchanged during long JJJJj^JJf 1 " 

 periods. Except in .the cases, therefore, in which organisms are of physical 

 transferred from one region to another, their evolution cannot in co 

 any large degree be due to the direct action of climate. A race 

 may gradually advance towards the equator and so, as the 

 generations pass, there may by successive steps be evolved a 

 constitution able to stand extremes of heat. But except under 

 such circumstances as these, the influence of climate is mainly 

 indirect. Severe cold may make it more difficult for one species 

 than for another to obtain food. The latter, therefore, will have 

 an advantage in a cold region and will probably oust the former. 

 Two species of grouse may be equally adapted to the physical 



