Darwin s Own Misgivings 357 



an eye incapable of seeing, and yet serving as a basis for 

 progressive improvement. Unless an animal could use such 

 an organ, it would have no advantage over others without 

 even a beginning. Darwin felt, however, that the evolution 

 of the eye from a rudimentary pigment spot sensitive merely 

 to varying intensities of illumination could come about 

 through natural selection by the accumulation of advan- 

 tages, given sufficient time. 



Another serious problem is the development of very 

 complex instincts, especially in the higher branches of the 

 different phyla. The social insects, for example, perform 

 most intricate acts that are definitely related to the survival 

 of the individual and of the species. These could not by 

 any conceivable means have been acquired in their entirety 

 at any one time, and yet could not be of survival value un- 

 less they were effective in their entirety. Here too, Darwin 

 thought, diligent search for intermediate stages might show 

 that instincts had evolved from the simpler to the more 

 complex by small stages at each of which natural selection 

 could have been effective. 



Finally the problem of intersterility of species was a 

 source of a great deal of concern. Although sterility is fre- 

 quently found among varieties of the same species, it is much 

 more common when different species are crossed; and the 

 offspring of specific crosses are themselves often sterile. 

 Since there was no definite knowledge regarding the mechan- 

 ism of reproduction and heredity in Darwin's time, it was pos- 

 sible to account for the sterility only by saying that there is 

 some disharmony between the reproductive mechanisms of 

 the two species. We do not know very much more today. But 

 we do know that in some cases the important factor lies in 

 a difference of chromosome numbers; and that in other cases 

 the chromosomes are so distinct in two species that there 

 cannot be the normal coupling which takes place after fer- 

 tilization. The utmost that Darwin's natural selection 

 could furnish toward the solution of this problem is the 

 suggestion that as a type diverges from the parental pattern 



