OTHER THEORIES OF SPECIES-FORMESTG 281 



in connection with the extensive group of lice (Mallophaga) that live 

 their whole lives buried among the feathers of birds or the hair of 

 mammals. These animals cannot fly and are quite effectively isolated 

 for Ufe upon a particular bird. They do, however, during the intimate 

 period of nesting, pass from parent to offspring, so that they may be 

 said to be isolated upon definite genetic lines. In the case, especially, 

 of birds like the eagle, a bird of long life and monogamous habits, the 

 parasite becomes as isolated as might be a race on a small island. The 

 result is that sometimes the lice of a single bird and its offspring are 

 of quite a distinct variety, which has become fixed by inbreeding until 

 a high degree of uniformity has been attained. Such an isolated 

 variety may be almost as distinct as a true species. Obviously in this 

 case, as in others, isolation must have had a real effect upon species- 

 forming quite apart from natural selection, except in so far as the unfit 

 variants have not survived. 



The writer's impression is that isolation as a factor in evolution 

 has been undervalued by the majority of writers on the subject. It is 

 a highly important and essential factor in the estabUshment of species. 

 If natural selection m.ay be said to be the prime factor in producing 

 adaptations, isolation may be said to be the prime factor in species 

 differentiation, guided only within moderate limits by natural selection. 



Biologic isolation. — The effects of this type of isolation are not 

 nearly so well established as are those of geographic isolation. Accord- 

 ing to this theory, differences in the rate of development or earliness 

 or lateness of the breeding season would serve to prevent certain 

 varieties from intercrossing. Only those individuals which were 

 sexually active simultaneously would mate, and individuals with 

 different breeding times and seasons would be isolated from one 

 another and would likely maintain the variations that arose in the 

 isolated stocks. The main weakness of this phase of isolation is, 

 however, that we have so little actual evidence that it is operative in 

 nature. 



Reproductive isolation. — A much more real type of isolation than 

 the last named is involved in reproduction. Several conditions may 

 arise of entirely distinct sorts that will tend to inhibit mating at ran- 

 dom. The first agency has been called "assortative mating" and 

 implies a sort of race feeling involving either a special attraction of like 

 for like, based on similarity of odors, colors, etc., or an antipathy 

 toward opposites or unlikes. The inhibition to general mating may 

 involve a mere mechanical lack of fit in certain organs necessary for 



