570 THE EVOLUTION OF LIFE. 



crossing, and divergence prevented; but that by the aid of 

 this kind of isolation, a uniform species may be differentiated 

 into two or more species, tliough its members continue to live 

 in the same area. 



Facts are assigned to show that slightly unlike varieties 

 may become unable to inter-breed either with the parent- 

 species or with one another. This mutual inferiority is not 

 of the kind we might expect. We might reasonably suppose 

 that when varieties had diverged widely, crossing would be 

 impracticable, because their constitutions had become so far 

 unlike as to form an umvorkable mixture. But there seems 

 evidence that the infertility arises long before such a cause 

 could operate, and that instead of failure to produce a work- 

 able constitution, there is failure to produce any constitution 

 at all — failure to fertilize. Some change in the sexual 

 system is suggested as accounting for this. That a minute 

 difference in the reproductive elements may suffice, plants 

 prove by the fact that when two members of slightly-diver- 

 gent varieties are fertilized by each other's pollen, the fer- 

 tility is less than if each were fertilized by the pollen of its 

 own variety; and where the two kinds of pollen are both 

 used, that derived from members of the same variety is pre- 

 potent in its effect over that derived from members of the 

 other variety. 



The writers above named contend that variations must 

 occur in the reproductive organs as well as in other organs; 

 that such variations may produce relative infertility in par- 

 ticular directions; and that such relative infertility may be 

 the first step towards prevention of crossing and estab- 

 lishment of isolation: so making possible the accumulation 

 of such differences as mark off new species. Without doubt 

 we have here a legitimate supposition and a legitimate infer- 

 ence, l^ecessarily there must happen variations of the kind 

 alleged, and considering how sensitive the reproductive sys- 

 tem is to occult influences (witness among ourselves the fre- 

 quent infertility of healthy people while feeble unhealthy 



