The Factors of Evolution 253 



while fertile, are said to have frequent miscarriages, and 

 after a few generations to be generally barren. In more ex- 

 treme cases, while fertilization is possible and development 

 may proceed for a time, the offspring of the cross do not at- 

 tain maturity. This is especially true of crosses between 

 widely divergent forms such as the salamander and the frog, 

 or the sea urchin and the starfish. 



In some instances isolation is effected by a difference in 

 the time of mating of different individuals. Ther'e are many 

 species of butterflies which have different color phases, to 

 which reference has already been made. These color phases 

 are apparently due to the time of year at which the eggs 

 are hatched, whether in spring, summer or autumn. It is* 

 probable that those butterflies which hatch at the different 

 seasons mate together, thus producing isolation of the dif- 

 ferent color phases of the same species, due to the different 

 hatching seasons of the eggs. Inhabiting the Kermadec Is- 

 lands northeast of New Zealand are two varieties of a spe- 

 cies of shore bird, which flock together but breed at different' 

 times, thus producing isolation between them. 



In other cases structural differences, notably of the external 

 sexual organs, such as occur in various species of insects and 

 other arthropods, effectually serve to prevent interbreeding; 

 while yet again structural differences in the germ cells them- 

 selves interfere with cross fertilization. 



Habitudinal isolation may be effected by the preference of 

 different groups of animals for different modes of life (dif- 

 ferences in food or habitat). Inhabiting the southern hemi- 

 sphere are several species of albatrosses, which mingle with 

 one another throughout most of their range, but breed in 

 separate localities. Professor Kellogg, whose name is famil- 

 iar to us all for his services to Belgium, some years ago made 

 a study of the bird lice, a group of wingless, biting insects 

 living on birds and mammals, similar in habit, though dif- 

 fering in structure from the "ugly, creepin' blastit wonner, 

 detested, shunned by saunt an' sinner," but immortalized by 

 Burns. In this study he found that while the similarity of 

 the environment in which these lice live tends to keep the 

 different species unchanged, nevertheless the isolation pro- 

 duced between groups of individuals living, it may be for 

 years on the body of the same bird, tends to fix the minor 

 variations which occur in all living things, and thus pro- 

 duce slight but noticeably distinct variations within the spe- 

 cies. 



The student of geographical distribution of plants and 

 animals recognizes as axiomatic the fact that the more widely 

 separated are any two regions of the earth 's surface, the more 



