INHERITANCE OF EYE-COLOUR IN GAMMARUS. ou 
second cross, all but one of the broods were hatched, the largest 
number being 49. 
F, generation. 
All the young of this generation were black-eyed, as was to be expected, 
in accordance with the Mendelian law that the offspring of Dominant 
mated with Recessive resemble the dominant parent in character. The 
further development of the law, that though the offspring are dominant 
in appearance, yet in constitution they are hybrid, could not be deter- 
mined until the next generation, the F,, appeared, the eye-colour alone 
not being a sufficiently accurate guide in distinguishing Pure Blacks from 
Hybrid Blacks. 
In order to make sure of each individual F, and to keep its history 
clear, all the F, broods were kept in separate bowls till mature, and then 
as each pair mated it was removed and records kept of all the matings, 
the young being counted and examined for eye-colour immediately after 
extrusion from the pouch. All the F, that reached maturity were tested 
and all proved Hybrid Blacks. 
The results for the eye-colour in the F, generation are given below. 
F, generation. 
The first idea was to take the F,, broods in order as they hatched to the 
number of 1000 young, and to find if the proportions held good—three 
black to one red. Seventy-four broods were taken in this way, the young 
numbering 586, 437 of which were black-eyed and 149 red-eyed, the reds 
therefore being very slightly in excess of the theoretical figure. These 
broods appeared during the summer months, when the animals mature 
more rapidly and have a much quicker succession of broods than in the 
lower temperature. As the numbers in the broods were decreasing, the 
adults dying off, and the whole of the stock looking unhealthy, it was 
thought well to strengthen it before continuing the experiment. A 
change of food was given, and plenty of mud from the ditches. 
It was then decided to pick out the three largest and strongest of the 
F, broods, and to count all the F, progeny produced by them. K, M, 
and N broods, which had matured under the healthier conditions, were 
chosen—K_ brood consisted of five males and nineteen females, M of 
nine males and six females, and N of fifteen males and seven 
females. 
The first 72 broods from these three families (from Oct. 22, 1914, 
to March 2, 1915) contained 1004 young, 753 black-eyed and 251 red- 
eyed, in the exact proportion, as will be seen, of 3 to 1. K family was 
represented by 43 broods, total number of young hatched 655, of which 
487 were Black and 168 Red; M family by 19 broods, 271 young, 204 
