28 EVOLUTION OF TO-DAY. 



causes there is much difference of opinion. The 

 causes of the variations must evidently be some- 

 thing connected with the organism and internal, or 

 something outside of it. We can suppose that all 

 variation is due to the action of external condi- 

 tions. Most animals have two parents and cannot, 

 of course, be like both. But aside from this fact, 

 the reason that a child differs from either of its 

 parents may be due to the fact, that it is never 

 under the same conditions as its parents. All varia- 

 tion would thus be dependent on the action of the 

 environment. But it is also possible to suppose that 

 the causes of variation are in the laws of the organ- 

 ism, and only indirectly related to the environment. 

 If this were true, the child might be like neither 

 parent, nor even midway between them, though the 

 circumstances were all alike. 



And right at this point lies the difference between 

 most of the modern theories of evolution. All theo- 

 ries are based fundamentally upon variations which, 

 we have seen, are so abundant. It is evident, there- 

 fore, that the particular theory which any one may 

 hold as to evolution, will depend upon his concep- 

 tion of the causes of and the laws governing these 

 variations. Assuming, for a moment, that new 

 species have arisen by the accumulation of such 

 variations, the question is resolved into another one, 

 what causes the variations ? The answers to this 

 question have been numerous. Darwin, while sup- 

 posing that all of the variations had a sufficient 

 cause, could discover no sufficient cause, and 

 thought that they were only indirectly related to 



