272, READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
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 life 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 asmallisland. 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 establishment of species. 
If natural selection may 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 
tmvolve a mere mechanical lack of fit in certain organs necessary for 
