LOSS OF EYE-PIGMENT IN GAMMARUS. 349 
occurred, all in one brood, two of which survived to produce offspring. 
All the animals used in the experiments were descendants of these two 
and the mutation has never occurred again. 
The loss of the white, extra-retinal pigment, on the other hand, has 
originated apparently independently on several occasions. It is dis- 
cussed in detail on p. 336. There seem to be some grounds for concluding 
that the loss of this pigment occurs when animals have been allowed to 
remain together for long periods and to interbreed promiscuously under 
somewhat unfavourable conditions as regards the quantity of water in 
which they are kept and the amount of food available. The loss of this 
pigment too, as already mentioned, seems sometimes to occur rather 
gradually and not suddenly as in the case of the change from black to 
red or the loss of the red. 
Quite colourless eyes have arisen independently from albinos by the 
loss of the white extra-retinal pigment, just in the same way that no- 
white eyes have arisen from normal red and black eyes. Cases of this 
kind have been discussed on pp. 336-338. 
A point of general interest which is somewhat strikingly illustrated 
by experiments described in this paper is the way in which the offspring 
of two abnormal, in this case degenerate, parents may themselves be 
quite normal in their characters, but are nevertheless capable of trans- 
mitting the abnormalities to their children. Such a case is that described 
on p. 333 (paragraph 2), where an Albino male (i.e. a male whose eye 
contained only the white extra-retinal pigment but neither black nor 
red inter-retinal pigment) was mated with a female Red No-white (i.e. 
one whose eyes contained only red inter-retinal pigment) with the result 
that all the young were black-eyed animals, normal in form and 
colour and indistinguishable on inspection from their wild ancestors 
(Plate VII, Figs. 4 and 5; Plate IV, Fig. 17). When, however, 
these or black-eyed animals of similar constitution are mated together 
the essential difference between them and the wild form comes out 
at once, all the abnormalities of the grandparents being reproduced 
in the grandchildren, and these abnormalities may even be combined in 
such a way that some of the grandchildren are more abnormal than the 
grandparents from whom they sprang. In the particular case mentioned 
the offspring consist, on the average, of 27 normal blacks, 9 black no- 
whites, 9 normal reds, 3 red no-whites, 12 normal albinos and 4 colour- 
less (albino no-whites). (Cf. Plate VI, Fig. 1.) 
Other results of a similar kind are recorded on p. 333 and the following 
pages. 
