4O EVOLUTION OF TO-DAY. 



are also found in individuals, the ease with which 

 crosses can be made varying very much in different in- 

 dividuals of the same species. Occasionally species 

 which are very difficult to cross produce, when they 

 do cross, offspring which are very fertile ; and species 

 which cross with great ease produce absolutely 

 sterile hybrids. And it is important to notice that 

 the best experimenters not unfrequently disagree in 

 regard to the very same species; some claiming 

 that given species are fertile when crossed, and others 

 claiming that they are sterile. 



Now all of these facts do show one thing, viz. : 

 that sterility of species, when crossed, is no definite 

 law. Sterility is a variable matter. It is not de- 

 pendent upon specific limits, nor upon the closeness 

 of the relations of the species crossed, but upon 

 some other factors not easily discovered. It is plain 

 from all these cases that sterility is not a bar which 

 rigidly separates species from each other. Natural- 

 ists have proved that this distinction between species 

 and varieties is not absolutely correct. Whatever 

 be our belief as to the origin of species, we cannot 

 believe that each was created with the bar of sterility 

 to separate it from others. 



Why, then, are species usually sterile when 

 crossed ? If this is not a created bar, how did it 

 arise ? We have seen that ordinary variations have 

 not yet been shown to give rise to sterility. From 

 various considerations most scientists are to-day in- 

 clined to accept the view that it is due largely if not 

 entirely to differences in the sexual organs indepen- 

 dently of differences in the rest of the body. Two 



