286 AN INTRODUCTION TO ZOOLOGY 



there by winds or carried on floating objects, have for a very 

 long time been isolated, and this isolation has so changed the 

 character of these animals that they are now recognized as 

 distinct species. There can be no doubt that the species origi- 

 nated from individuals from the mainland, and that the most 

 important factor in their transmutation has been geographical 

 isolation (274). 



PHYSIOLOGICAL ISOLATION. Internal differences may cause 

 physiological isolation, a factor in the origin of species as impor- 

 tant probably as geographical isolation. Physiological, or, as it is 

 sometimes called, sexual isolation is caused by " the influence 

 of some variation tending to make difficult or impossible wholly 

 free and miscellaneous mating or breeding inside of a species. 

 This variation may be of purely physiological character or may 

 be a structural one: that is, the hindrance to mating may be one 

 of instinctive feeling, a ' race-feeling ' depending on an antipathy 

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

 tion of the copulatory organs making such mating difficult, or 

 even a modification of the egg or the spermatozoids making fer- 

 tilization difficult. It is a well-known fact thai: numerous 

 varieties of domesticated animal speHes rarely breed together, 

 although quite able to, and provided with, full opportunity. On 

 the other hand, animals of different species which in Nature 

 rarely or never breed together may, if kept long in confinement, 

 as in zoological gardens, mate 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 hybridizing " (250, p. 245). In many 

 cases, if interbreeding does occur, the hybrid offspring are sterile, 

 e.g. the mule produced by a cross between a female horse and a 

 male ass. By physiological isolation, then, groups of individuals 

 are formed within the same region. The result is in every way 

 similar to segregation by geographical barriers. 



