32 ORGANIC EVOLUTION 



fully observe relative degrees of fertility and select as pro- 

 genitors for the several races individuals which would not 

 readily breed with others than those of their own race. As 

 a matter of fact breeders have not cared to produce infertile 

 races and have not done so. There seems little doubt that 

 they could have done so. 



Mutual infertility between certain individuals may often 

 in nature have been the starting-point in the divergence 

 which has resulted in the establishment of new species. 

 This point will be discussed farther on. 



Another objection which has been urged against the 

 efficiency of natural selection as a factor in evolution is the 

 fact of the apparent uselessness of some of the character- 

 istics of different species, both animals and plants. If a 

 character is useless how can it have been developed by 

 / natural selection, which operates only to perpetuate char- 

 acters which aid their possessors in the struggle for exist- 

 ence ? First let us ask, are useless characters really found ? 

 Apparently they do occur, but much less frequently than 

 we would at first thought suppose. Careful study often 

 shows that structures or habits apparently useless are of 

 real value to their possessors. One would find it difficult 

 to give an instance of an organ or characteristic which he 

 is sure is of no value to the plant or animal in which it is 

 found. Yet we could probably find such instances. Many 

 are familiar with the beautiful markings on the shells of 

 diatoms, a group of microscopic Alga, or with the beauti- 

 fully regular skeletons of many other microscopic animals 

 and plants. These shells and their markings are often 

 of elaborate pattern (Plate 21); they are regular in their 



