150 Evolution and Adaptation 
following years, to fertilize reciprocally M7. dongifiora with the 
pollen of M7. 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 
