188 E VOL UTION AND DOGMA. 



as is so often contended, that species have been 

 endowed with sterility in order thereby to prevent 

 their becoming confounded in nature, why is it that 

 we find so many exceptions to what is said to be an 

 invariable law? "Why," asks Darwin, "should the 

 sterility be so extremely different in degree when 

 various species are crossed, all of which we must 

 suppose it would be equally important to keep from 

 blending together? Why should the degree of 

 sterility be innately variable in the individuals of 

 the same species ? Why should some species cross 

 with facility, and yet produce very sterile hybrids ; 

 and other species cross with extreme difficulty, yet 

 produce fairly fertile hybrids? Why should there 

 often be so great a difference in the result of a re- 

 ciprocal cross between the same two species? Why, 

 it may even be asked, has the production of hybrids 

 been permitted ? To grant to species the special 

 power of producing hybrids, and then to stop their 

 further propagation by different degrees of sterility, 

 not strictly related to the facility of the first union 

 between their parents, seems a strange arrange- 

 ment." ' 



To show to how great absurdities a too strong 

 insistence on physiological species, as an absolute 

 criterion as to what is a true species and what is 

 but a simple variety, may sometimes lead, I need 

 only refer to a large number of groups of flowers, in 

 which individuals of a given species can be more 

 easily fertilized by pollen from a different plant, or 

 even by the pollen of a different species, than by 



1 " The Origin of Species," vol. II, p. 17. 



