130 REDUCTION OF VARIABILITY. 



to diminish even in the absence of every selection This makes 

 it unnecessary to assimie that every quality for whi6h we find a 

 group stable, must be necessarily useful, and have selection- 

 value 



It is evident that no species can ever become pure for a really 

 harmful character. But we do not need to beheve that all the 

 diverse forms of life have become as we now find them, because 

 they have each fitted themselves to special conditions of Ufe. 

 The very fact, that thousands upon thousands of species exist 

 and survive on earth, all different in hundreds of the most fun- 

 damental characters, shows, that the fitness of an organism is 

 only a matter of finding the exact niche it can fiU. 



It is hard to conceive of an imaginary organism, composed of 

 parts taken from the most diverse species, which would be in- 

 capable of existing under some set of conditions or other. In a 

 world where an echidna, a tapeworm, an elephant, a cassowari 

 and a bee all find their living, surely there would be place for a 

 healthy unicorn. 



There is nothing which binds ^an organism to its environ- 

 ment so much, as just its adaptation to it. Young individuals 

 wiU live in the same environment as their parents only, if they 

 are equally well-fitted to live there. If they are different, so 

 that they do not fit there, they wiU happen to find another en- 

 vironment into which they do fit, or else they will perish, just 

 like the individuals which are like their parents but drift into 

 the wrong environment. Think of the series of rare lucky cir- 

 cimistances which the favored few tapeworms must encoim- 

 ter, before they are safely housed in the inside of a suitable 

 host. At every stage the chances are one to several thousand, 

 that their right environment will be found. In aU the other 

 cases the animal perishes. Every sort of animal or plant which is 

 now Uving, is adapted to its environment, but nothing forces 

 us to assiune that the ancestral forms of any organism were 

 adapted to the same enviromnent. 



The result of crossing, wherever the forms crossed, differ in 

 several genes, is a great genotypic diversity, a diversity which 



