348 E. J. ALLEN AND E. W. SEXTON. 
eyes of these are quite colourless. The figures for these are given under 
Section III, Summary (pp. 346 and 347, b, d and e). 
In addition to the colourless-eyed animals obtained by crossing albino- 
and no-whites, the colourless eye has arisen independently as a mutation. 
Colourless-eyed animals mated together give all colourless-eyed off- 
spring. The figures for these are given under Section III, Summary 
(p. 345. AN AN). 
GENERAL CONSIDERATIONS. 
The phenomena described in the present paper show a progressive 
degeneration of the eye of Gammarus, taking place in a series of definite 
steps or stages, each of considerable magnitude. In the end we see the 
entire loss of the eye-pigment, together with a broken and irregular 
arrangement of the ommatidia and a great reduction in their number. 
We only need to imagine the continuation of this process for a few 
further steps, and we should reach the complete absence of eyes found in 
those blind genera of Amphipoda, which live in subterranean waters. 
There is no direct proof that the change from the black eye-pigment 
of the wild animal to the red pigment, which occurred as a mutation in 
the eyes of the animals first used in the experiments, is due to the loss 
of a factor, but it seems not improbable that this may be the case. It is 
clear, at any rate, that these degenerative changes—the change from 
black pigment to red pigment, the entire loss of the coloured retinal pig- 
ment, the loss of the white extra-retinal pigment, and the degeneration 
in the form of the eye—all take place in exact conformity with Mendel’s 
Law. The only feature which may at first sight seem to show a diver- 
gence from this law is the more gradual process of degeneration of the 
white extra-retinal pigment, which gives rise to what we call the “ no- 
white ” eyes described in Section III. This, however, may perhaps be 
capable of explanation by supposing that the loss of the pigment takes 
place in a series of steps, instead of in one single step. 
The experiments recorded throw little or no light on the question of 
the conditions under which mutations arise or of the causes which give 
rise to mutations. The mutation of red eye-pigment has arisen once only 
in the whole course of the work and then after the animals had been kept 
under Laboratory conditions for only 2 generations. 
The complete loss of the inter-retinal coloured pigment, giving rise 
to the “ albino ” eye, was first seen in one brood belonging to a particular 
family as described on p. 275, the female parent being from stock which 
had been living under laboratory conditions for over 3 years, and the 
male parent also from stock which had been in the laboratory for several 
generations. Out of 733 offspring of the same family, 4 with albino eyes 
