OTHER THEORIES OF SPECIES-FORMING. 245 



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

 for that or any other reason, ?. 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 wholly free 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 1 ' 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 



Physiological S p ec i es _f orrnm o- bv Romanes 15 and others, 

 selection, 



Romanes and Hutton believe that a progressive 



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



