66 EVOL UTION AND NATUEAL THEOL OGI. 



VII. "The objection drawn from the .phy- 

 siological difference between c species ' 

 and ' races ' still exists unrefuted." 



This is indeed the great stumbling-b]ock to 

 all theories of Evolution, nor has any perfectly 

 satisfactory explanation yet been given of the 

 sterility of most species when crossed, and the 

 fertility of most varieties. Darwin * however, 

 in his usual masterly manner, has brought for- 

 ward a great number of facts and arguments to 

 prove that this rule is not absolute, and varies 

 very much in its operation. He thinks that the 

 sterility of hybrids is due in many cases to the 

 early death of the embryo ; but this explanation 

 is neither very clear, nor very satisfactory, and 

 the following may be suggested as perhaps the 

 true one. 



It is well known that crossing promotes 

 reversion, and that many plants and animals are 

 normally hermaphrodite ; but it is certain that 

 hybrids frequently show more or less tendency 

 to hermaphroditism, and if this effect is due to 

 reversion, it will at once satisfactorily account 

 for hybrids being frequently sterile, even where 



* " Origin of Species," ch. viii. 



