On Certain Factors of Evolution. 815 
out the cave region. I have never found a stray Asellus even 
partly bleached and with diminished eyes in any caves, nor seen 
such specimens in collections made by others, though they may 
yet be found. Whether living in caves or wells fed by subter- 
ranean streams, the bleached, eyeless, or nearly eyeless, forms breed 
true to their type, and show no signs of intercrossing with luco- 
philous forms. 
Should, however, these cave forms be placed in such circum- 
stances as to be able to mix or intercross with their epigæan allies, 
which are in all probability the very species to which they owe their 
origin, there would with little doubt be a constant tendency to 
revert to the ancestral eyed forms, and we should constantly find 
certain individuals with visual organs better developed, and with a 
darker integument, serving as connecting links, Such links may 
have been common enough when the caves were first formed and 
colonized, and in some species, as Pseudotremia cavernarum, they 
frequently occur at the present time, but, as a rule, owing to lon 
isolation or seclusion, and the consequent impossibility of intercross- 
ing, they are now rare. 
But as circumstances are now, the total darkness, the temperature, 
the degree of dryness or the moisture, and other physical conditions 
remaining the same, the eave fauna is almost completely isolated 
from that of the upper world; indeed, far more so than the deep- 
sea fauna of the ocean or of lakes, or the faunas of deserts or of the 
polar regions, or the alpine inhabitants of lofty mountain summits. 
We thus realize that isolation may be a not unimportant factor in 
securing permanence of type, after the typical characters have once 
been established through adaptation and heredity. 
After reflecting upon the influence of isolation upon cave animals 
as securing permanence of varietal, specific, and generic characters, 
one is led to realize as never before the importance of geographical 
isolation in general as a faetor in preventing variation after the 
organisms have once become adapted to their peculiar environment, 
whether dependent on temperature, soil, humidity, or dryness, the 
absence of light, or any other appreciable characteristic in their 
Surroundings. We know also that the existing desert, deep-sea, 
and polar faunas are the product of Quaternary times; that they 
Were nearly contemporaneous in origin with the cave faune, 
