LOSS OF EYE-PIGMENT IN GAMMARUS. 291 
by further breeding experiments whether or not the IF, offspring contain 
blacks with one dose of black. If a one-dose black be mated with another 
one-dose black the offspring will be all black, if mated with a two-dose 
black they will be all black, but if mated with a hybrid black (black 
carrying red) the offspring will contain some red, as we have seen in 
considering the cross VI.A. IL.A. 
If we mate together the blacks of the F, generation we obtain in F; 
some broods which contain red-eyed animals, others which contain only 
blacks. The parents of the broods containing red eyes will either be two 
hybrids, or a hybrid and a one-dose black, if the latter exists. If we 
cross-mate the parents of a number of such broods, in as many different 
ways as possible, we ought eventually to bring two one-dose blacks 
together, in which case we should get all black offspring. 
A second test will be as follows. If a one-dose black be mated with a 
red it will, according to theory, give blacks and reds in equal numbers, 
behaving in exactly the same way as a hybrid black. If therefore we 
take blacks which give red offspring when mated with red, and mate 
them together, we ought, if the one-dose black exists, to obtain some broods 
which give all black as the result of two one-dose blacks coming together. 
By mating together blacks tested with reds in this way, and blacks 
tested with other blacks and giving red in their broods, we have a further 
opportunity of bringing together two one-dose blacks (if they exist). 
These tests have been applied, but we have not been able to find 
any one-dose blacks, all those tried proving ordinary hybrid blacks, 
giving both red and black offspring. (See list of cross-matings, p. 303.) 
Cross C. F.2. GENERATION. BLACKS. 
The following lists show (1) the constitutions of all the blacks of these 
broods which have been tested (see Plate II, VI.A and VI.C); and (2) 
the results of the cross-matings made with blacks which had given some 
red offspring when mated with either red or black mates :— 
(1) The Black-eyed young, showing their constitution and the matings 
by which they were proved. 
VI.A.l.a. Male, Black carrying the factor for Red only (Plate IV, Figs. 
2, 4 and 5). 
Matings :—(1) with female from Pure Red Stock; 8 
young, 5 Black, 3 Red ; 
\(2) with female | of its own brood, (B+ R+ 
A); 61 young, 49 Black, 12 Red ; 
(3) with female of VI.C.l.d. (B+R+A); 42 
young, 30 Black, 12 Red. 
