MR. G. J. ROMANES ON PHYSIOLOGICAL SELECTION. 351 



that in many cases, even when tamed in their own countries, 

 allowed freedom, fed on their natural food, and so forth, animals 

 become absolutely sterile. Moreover, so delicately is the repro- 

 ductive system balanced in respect of variability, that sometimes 

 it will change in the direction of sterility and sometimes in the 

 opposite direction of increased fertility, under a change of con- 

 ditions the same in kind, but different in degree. Lastly, in 

 numberless individual cases variability occurs in either of these 

 two opposite directions without any assignable reason at all, or, 

 in Mr. Darwin's language, spontaneously. So that, on the 

 whole, we must accept it as a fact that the reproductive system, 

 both in plants and animals, is preeminently liable to vary, and 

 this both in the direction of sterility and in that of increased 

 fertility. Indeed, Mr. Darwin goes so far as to say : " It would 

 appear that any change in the habits of life, whatever these 

 habits may be, if great enough, tends to affect, in an inexplicable 

 manner, the powers of reproduction." And he adds this im- 

 portant qualification : " The result depends more on the consti- 

 tution of the species than on the nature of the change ; for 

 certain whole groups are affected more than others ; but excep- 

 tions always occur, for some species in the most fertile groups 

 refuse to breed, and some in the most sterile groups breed 

 freely." 



Now, having regard to all these delicate, complex, and for the 

 most part hidden conditions which determine this double kind of 

 variation within the limits of the reproductive system, there can 

 be no diffieulty in granting that variations in the direction of 

 greater or less sterility must frequently occur in wild species. 

 Probably, indeed, if we had any means of observing this point, 

 we should find that there is no one variation more common ; but 

 of course, whenever it arises, whether as a result of changed 

 conditions of life, or, as we say, spontaneously, it immediatelv 

 becomes extinguished, seeing that the individuals which it 

 affects are less able, if able at all, to propagate the variation ; or, 

 if the variation should extend to all the individuals of a species 

 under a change of environment, that the species would become 

 extinct. 



Let these three points, then, be clearly kept in mind : 1st, that 

 when a section of any species is cut off by geographical barriers, 

 or by migration, from intercrossing with its parent form, it tends 

 to run into new varieties, and so eventually to develop new 



