150 Evolution and Adaptation 



following years, to fertilize reciprocally M. longiflora with the 

 pollen of M.jalapa, and utterly failed." 



A formal interpretation of this difference can be easily 

 imagined. The infertility in one direction may be due to 

 some physical difficulty met with in penetrating the stigma, 

 or style. For instance, the tissue in one species may be too 

 compact, or the style too long. Pfliiger, who carried out a 

 large number of experiments by cross-fertilizing different 

 species of frogs, reached the conclusion that the spermatozoa 

 having small and pointed heads could cross-fertilize more 

 kinds of eggs, than could the spermatozoa with large blunt 

 heads. This is probably due to the ability of the smaller 

 spermatozoa to penetrate the jelly around the eggs, or the 

 pores in the surface of the egg itself. But there are also 

 other sides to this question, as recent results have shown, for, 

 even if a foreign spermatozoon can enter an egg, it does not 

 follow that the development of the egg will take place. 

 Here the difficulty is due to some obscure processes in the 

 egg itself. Now that we know more of the nicely balanced 

 combinations that take place during fertilization of the egg, 

 and during the process of cell division, we can easily see that 

 if the processes were in the least different in the two species 

 it might be impossible to combine them in a single act. 



" Now do these complex and singular rules indicate that 

 species have been endowed with sterility simply to prevent 

 their becoming confounded in nature ? I think not. For 

 why should the sterility be so extremely different in degree, 

 when various species are crossed, all of which we must sup- 

 pose it would be equally important to keep from blending 

 together ? " 



" The foregoing rules and facts, on the other hand, appear 

 to me clearly to indicate that the sterility both of first crosses 

 and of hybrids is simply incidental or dependent on unknown 

 differences in their reproductive systems; the differences 

 being of so peculiar and limited a nature, that, in reciprocal 



