OTHER THEORIES OF SPECIES-FORMING. 243 



From the foregoing it is obvious that geographical isola- 

 tion is a proved, effective, and widely applied species-form- 

 ing factor. So much cannot be said for biologic 

 Biologic an( j sexlla j isolation. 11 The actual existence of 



isolationi 



such isolation is not proved by any mass of evi- 

 dence based on observation, although the theory is by no 

 means pure speculation ; nor are the results of such isola- 

 tion clearly indicated, although a certain amount of observa- 

 tion and experimental evidence can be adduced to show 

 them. The different phases of isolation, not geographic, 

 called by different names, as biologic, physiolpgic, sexual, 

 and morphologic isolation, all have reference to some sort 

 of segregation of individuals of the same species into groups 

 inside of each of which mating takes place, and among 

 which little or no cross-breeding occurs, because of varying 

 habits, or unusual sexual aversion or attraction, or physio- 

 logical or morphological variation affecting mating. For 

 example, to take first a sort of combination of geographic 

 and biologic isolation, Plate 12 points out that there are 

 twelve species of albatrosses in the southern hemisphere, of 

 which nine or ten belong to the Australian zoo-geographical 

 realm and intermingle throughout most of their range. At 

 breeding time, however, these different species become 

 segregated in restricted and separate localities so that mating- 

 is always accomplished among different individuals of the 

 same species, although hybridisation could doubtless obtain 

 successfully among these closely related albatross forms. 

 Thus the species characters are kept pure ; the species dis- 

 tinct. 



An example of pure biological isolation and one within 



a single species (which is the sort of isolation we are more 



An example of interested in) might be produced in the follow- 



LltiSght in wa y- We know of numerous species of 

 work. butterflies which appear in different seasons of 



the year in different colour-pattern. This is not a colour 



