On Certain Factors of Evolution. 813 
of rearing normal, eyed Gammari in darkness, and refers to Hum- 
bert’s statement that in the greater depths of Lake Baikal, with 
an increase in depth of their habitat, there is an increasing lack 
of development of the eyes in some Gammaride. Fries also 
states that he himself had previously observed a decrease in the 
pigment of the eyes in young examples of Gammarus pulex living 
in darkness.” 
Here should be cited the observations of Anton Stecker, who 
states that Chermes, usually said to be eyeless, has rudimentary eyes, 
represented by clear, somewhat transparent spots, the chitin forming 
them being devoid of the granulations covering the rest of the 
shield, 
“ Each cornea is supplied by a large and well-developed optic 
nerve, proceeding from an optic ganglion in connection with the 
brain. But the layer of crystalline rods was wholly absent. About 
30 to 35 per cent. of the specimens of Chermes cimicoides examined 
possessed these eye-spots; in the remaining 65 to 70 per cent. they 
were absent, as well as the optic nerves; while there was only one, 
or even no recognizable rudiment of an optic ganglion. He also 
found that the offspring of parents, both of which had eyes, were 
themselves provided with them; but that if either the father or 
the mother were blind, the young were also blind, having at most 
a feeble indication of optic lobes. Dr. Stecker considers this a most 
instructive case of the gradual atrophy of an organ by disuse owing 
to the influence of changed conditions. There can be little doubt 
that the ancestors of Chermes possessed well-developed eyes; the 
first steps in the retrogressive process was the loss of the cornea 
and cones, the optic nerve and ganglion remaining after the true 
percipient apparatus had gone.” ! 
Here is a fertile field for careful and long-continued observa- 
tions on animals reared in different degrees of darkness. Such 
experiments will afford a crucial test of the theory of rapid evo- 
lution of genera and species due to sudden changes in the environ- 
ment, 
It is evident that physiological experiments are needed as well 
as embryological studies, to throw further light on the origin of 
cave animals, The blind-fish, blind crayfish, and Cæcidotæa, 
1 Morp. Jahrbuch, iv., 279, 1878; Journ. Roy. Mier. Soc., ii., 146, 1879. 
