Variation and Natural Selection. 105 



account of sterility. But although it could not be swamped 

 by intercrossing, still, if it arose sporadically, here a case, 

 there a case, and so on, the chances would be enormously 

 against the perpetuation of the variety, unless some co- 

 operating mode of segregation aided in bringing together 

 the varying individuals. If, for example, there were a 

 segregation of these variants in a particular habitat — all 

 the variants meeting in some definite locality for breeding 

 purposes ; or if there were a further segregation through 

 mutual preferences ; or if, again, there were a further 

 segregation in time ; the variety might obtain a firm 

 footing. But without these co-operating factors it is clear 

 that if one male and one female in a hundred individuals 

 varied in this particular way, the chances would be at 

 least forty- nine to one against their happening to mate 

 together. 



It is interesting to note that almost the only particular 

 example given by Mr. Piomanes in illustration of his theory 

 is one that involves the co-operation of one of these further 

 segregation-factors. Suppose, he says, the variation in 

 the reproductive system is such that the season of flower- 

 ing or of pairing becomes either advanced or retarded. 

 This particular variation being inherited, the variety breed- 

 ing, let us say, in May, the parent species in July, there 

 would arise two races, each perfectly fertile within its own 

 limits, but incapable of crossing with the other. Thus is 

 constituted " a barrier to intercrossing quite as effectual 

 as a thousand miles of ocean." Yes ! a time-barrier instead 

 of a space-barrier. The illustration is faulty, inasmuch as 

 it introduces a mode of segregation other than that in 

 question. I think it very improbable that differential 

 fertility alone, without the co-operation of other segregation- 

 factors, would give rise to separate varieties capable of 

 maintaining themselves as distinct species. 



That distinct species are generally mutually infertile, 

 or more frequently still, that their male offspring are 

 sterile, is, however, an undoubted fact. But there are, 

 exceptions. Fertile hybrids between the sheep and the 



