816 On Certain Factors of Evolution. 
though the deep-sea faunæ may date from the cretaceous period. 
Finally, I may quote from Darwin’s “Origin of Species” the 
following extract, which applies (though he did not make it appli- 
cable to any special case) with peculiar force to cave fauna: “ If, 
however, an isolated area be very small, either from being sur- 
rounded by barriers, or from having very peculiar physical condi- 
tions, the total number of the inhabitants will be small, and this 
will retard the production of new species through natural selection, 
by decreasing the chances of the appearance of favorable individual 
differences ” (Fifth edition, New York, p. 105). 
Heredity.—The action of this all-powerful factor in evolution is 
as constant in the underground world, and as difficult to compre- 
hend in considering cave life, as that of the upper regions. It begins 
to act, of course, with the earliest generations, and continues to act 
with, so to speak, increasing force and precision as time goes on and 
the characteristics induced by a life in total darkness becomes more 
and more fixed. 
It is evident that heredity has acted longest in those insects, such 
as the species of Anophthalmus and Adelops, whose larve are 
lacking in all traces of eyes and optic nerves and lobes. Heredity 
has here acted with unabated force throughout every stage of the 
metamorphosis ; and, it will be a matter of great interest to ascer- 
tain whether any traces of the eyes"*may be met with in the embryo 
of these forms, 
On the other hand, in those Arthropods in which the brain and 
optic nerves have persisted, with rudiments of the eyes (eg, 
Orconectes), where the eyes are larger in the young, it would seem 
as if heredity had been acting through a shorter period, and conse- 
quently, so to speak, with less momentum. 
In the case of Machærites, in which the females only of certain 
species are said to be blind, while the males have well-developed 
eyes, we have an apparent exception to the continuous action ° 
heredity ; an exception paralleled, however, by animals living ” 
the upper world, such as Termes, whose workers and soldiers are 
eyeless, though the males and females are eyed. They per haps 
are twilight species rather than inhabitants of totilly dark localities 
in caves, and those living in twilight may intercross with those 
