242 EVOLUTION OF TO-DAY. 



between species, which must have existed according 

 to this theory, are not forthcoming in the fossil 

 records, requiring thus a great burden to be thrown 

 upon the imperfection of this history. Many spe- 

 cies are distinguished by features of no use to them, 

 and these never could have been developed by 

 natural selection alone. It is mathematically demon- 

 strable that single variations could not give rise to 

 permanent varieties, but would tend to disappear in 

 successive generations 'by crossing, and not to in- 

 crease as would seem necessary on the theory. It 

 is therefore absolutely necessary to assume that 

 variations occur in numerous individuals simultane- 

 ously, and to find a reason why this should be so, 

 before the question is satisfactorily answered. The 

 evidence which indicates that we must look for 

 some internal factor, is conclusive. The difficulty of 

 accounting for the beginning of organs without some 

 such assumption, the difficulty of seeing how minute 

 variations could be of enough importance to be 

 preserved by natural selection, the independent 

 development of similar highly developed organs in 

 widely separated animals, all serve to strengthen the 

 belief that we must look to the organism itself, in 

 part, for the laws governing the origin of species. 

 Finally, when we consider that any variation, in 

 order to be preserved, must be of enough import- 

 ance to regulate the life or death of the species, it 

 becomes almost impossible to see how various struc- 

 tures of morphological species, or how the various 

 castes of sterile insects could ever have been devel- 

 oped upon this principle alone. 



