OTHER THEORIES OF SPECIES-FORMING. 245 



be a variance in sexual maturity, a change in breeding time 

 for that or any other reason, a tendency on the part of cer- 

 tain individuals to live more or less concealed in holes, under 

 stones, etc., changes in food-habits, as the gradual going 

 over of some individuals of a plant-feeding insect species 

 from the old food-plant to a new one, or the tendency within 

 an omnivorous species for groups to restrict themselves to 

 certain specific foods : all such variations might lead to pos- 

 sible biological isolation. 



By sexual isolation authors usually refer to the influ- 

 ence of some variation tending to make difficult or impossi- 

 Sexual ^ e who^y f re e and miscellaneous mating or 



isolation, breeding inside of a species. This variation 



may be of purely physiological character or may be a struc- 

 tural one : that is, the hindrance to mating may be one of 

 instinctive feeling, a "race-feeling" depending on an antipa- 

 thy to odour, to age, to appearance, etc., or may be a slight 

 modification of the copulatory organs making such mating 

 difficult, or even a modification of the egg or the spermato- 

 zoids making fertilisation difficult. It is a well-known fact 

 that numerous varieties of domesticated animal species 

 rarely breed together, although quite able to, and provided 

 with full opportunity. On the other hand, animals of differ- 

 ent species which in Nature rarely or never breed together 

 may, if kept long in confinement, as in zoological gardens, 

 mate 14 and produce young. In each case there seems to be 

 question of a "race- feeling" ; in the first case a sexual 

 aversion keeping apart individuals of the same species, in 

 the second the breaking down of race-feeling that in Nature 

 has sufficed to prevent hybridising. This might be termed 

 physiological isolation, or, indeed, physiological selection, as 

 it has been called, and given much credit for 



sel P ect y ion 10giCal species-* 01 " 1 1 ^ b y Romanes 15 and others. 



Romanes and Hutton believe that a progressive 



infertility results in this way (and also by the way referred 



