232 CAUSES OF THE STERILITY [CHAP. IX. 



now, what is there which could favour the survival of those indi- 

 viduals which happened to be endowed in a slightly higher degree 

 with mutual infertility, and which thus approached by one small 

 step towards absolute sterility ? Yet an advance of this kind, if 

 the theory of natural selection be brought to bear, must have in- 

 cessantly occurred with many species, for a multitude are mutually 

 quite barren. With sterile neuter insects we have reason to believe 

 that modifications in their structure and fertility have been slowly 

 accumulated by natural selection, from an advantage having been 

 thus indirectly given to the community to which they belonged 

 over other communities of the same species; but an individual 

 animal not belonging to a social community, if rendered slightly 

 sterile when crossed with some other variety, would not thus itself 

 gain any advantage or indirectly give any advantage to the other 

 individuals of the same variety, thus leading to their preservation. 

 But it would be superfluous to discuss this question in detail ; 

 for with plants we have conclusive evidence that the sterility of 

 crossed species must be due to some principle, quite independent 

 of natural selection. Both Gartner and Kolreuter have proved 

 that in genera including numerous species, a series can be formed 

 from species which when crossed yield fewer and fewer seeds, to 

 species which never produce a single seed, but yet are affected by 

 the pollen o* certain other species, for the germen swells. It is 

 here manifestly impossible to select the more sterile individuals, 

 which have already ceased to yield seeds ; so that this acme of 

 sterility, when the germen alone is affected, cannot have been 

 gained through selection ; and from the laws governing the various 

 grades of sterility being so uniform throughout the animal and 

 vegetable kingdoms, we may infer that the cause, whatever it may 

 be, is the same or nearly the same in all cases. 



We will now look a little closer at the probable nature of the 

 differences between species which induce sterility in first crosses 

 and in hybrids. In the case of first crosses, the greater or less 

 difficulty in effecting an union and in obtaining offspring appar- 

 ently depends on several distinct causes. There must sometimes 

 be a physical impossibility in the male element reaching the ovule, 

 as would be the case with a plant having a pistil too long for the 

 pollen-tubes to reach the ovarium. It has also been observed that 

 when the pollen of one species is placed on the stigma of a dis- 

 tantly allied species, though the pollen-tubes protrude, they do 

 not penetrate the stigmatic surface. Again, the male element 

 may reach the female element but be incapable of causing an 

 embryo to be developed, as seems to have been the case with some 

 of Thuret's experiments on Fuci. No explanation can be given of 

 these facts, any more than why certain trees cannot be grafted on 



