204 Mr Heape, Note on the influence of extraneous forces 
same cock with one of G’s hens is responsible for 3 nests, and 
13 eggs, which produced 11 cocks and 2 hens. 
Thus to compare the two. In G’s aviary a more regular tem- 
perature was kept during the breeding time ; there was more light 
and sun in the room ; less food was given ; hatching and moulting 
took place earlier as a rule ; only about half the percentage of loss 
was experienced ; and from the nests in which all eggs were 
hatched, the proportion of cocks produced was more than three 
times the proportion obtained in N’s aviary. 
It seems clear that while Ns birds were highly fed, possibly 
with the idea of counteracting the uncongenial aspect of the 
aviary, G’s birds attained breeding condition earlier with less fond. 
While N’s birds laid 44 eggs per nest, G’s laid 4*2 ; but 
although N’s birds laid more eggs a very much larger percentage of 
them did not hatch, and it may be concluded G’s birds had 
much the greater reproductive power. 
From a comparison of the dates of nesting and of the moult- 
ing of the cocks, I am inclined to think the large percentage of 
loss among N’s eggs is principally due to the cocks. 
It might be argued that the marked tendency of G’s birds to 
produce an excess of cocks was due to the particular strain of birds 
he bred from. It is true that when N bred from a pair of G’s 
birds the produce hatched was at the rate of 166 6 cocks per 100 
hens (more than double the average proportion he gets) ; but 
during that year the birds were subject to special feeding and that 
same year another pair of N’s own birds hatched eggs in the 
proportion of 200 cocks per 100 hens (a proportion which had only 
three times been equalled by any pair of his birds during the 
5 preceding years), and in a third pair, also of N’s birds, 400 cocks 
per 100 hens was the proportion hatched (a unique experience for 
him). Thus under N’s treatment the birds obtained from G did 
not hatch so high a proportion of cocks as did other birds of his 
own rearing that particular year. 
On the other hand, when G bred from a pair of birds obtained 
from N, the proportion hatched was at the rate of 900 cocks per 
100 hens, and when the cock of this pair was mated with one of 
his own hens 550 cocks per 100 hens was the proportion actually 
produced. 
So far as the evidence goes therefore it does not appear that 
the high proportion of cocks hatched in G’s aviary is due to the 
strain of his birds, but to some other factor or factors which 
influenced all birds kept by him, a conclusion which is borne out by 
N’s experience during 1893. 
In a recent paper “ On the proportion of the sexes in Dogs,” 
published in the Proceedings of this Society, I have suggested that 
in all animals in which only a limited number of ovarian ova 
