1878.] Doctrine of Development . 457 
leads to the conclusion that permanently unpaired birds are 
very scarce ; so that, speaking broadly, every bird finds a 
mate and breeds.” 
Mr. Morant also remarks that there must exist somewhere 
“ an establishment for unmarried female falcons.” 
Mr. Wallace very justly argues that this fad must coun- 
teract the effects, if any, of Sexual Selection. But it is 
scarcely less hostile to the action of Natural Selection. 
Granting that the pairs, as first formed, are composed of the 
strongest and most vigorous males and of the finest and 
healthiest females. But after a short time of the non- 
seleCtive slaughter carried on, if not by man, yet by hawks, 
ravens, wild cats, weasels, snakes, and other bird-destroyers, 
the rejected of either sex find themselves mated, and of 
course become parents, substantially to as great a degree as 
their more favoured rivals. It may of course be contended 
that this indiscriminate slaughter falls equally upon the 
mated and the unmated. We doubt the correctness of this 
supposition : birds in the various operations connected with 
nest-building, hatching, and feeding their young, have to 
expose themselves necessarily more to danger than their 
bachelor and spinster neighbours. Among the lower ani- 
mals, as well as among mankind, the pleasures and advan- 
tages of married life have, it seems, to be paid for. 
Hence, without at all seeking to deny the existence and 
working of Natural Selection as a force effecting modifica- 
tions in organic life, which may often extend to the forma- 
tion of what we call species, we feel bound to admit that 
its influence is checked and modified in a variety of 
manners. 
In Mr. Wallace’s work another interesting question is 
discussed with results which further strengthen us in the 
belief that Evolution must have other — and probably more 
powerful — causes, and has at all events not always been 
effected by uniform and imperceptible gradations. 
We are here reminded that the progressive development 
of the senses — a point scarcely as yet sufficiently investi- 
gated — is one of the most efficient ways in which animals 
may become modified in harmony with varying circum- 
stances. An individual bird or beast, if possessing sharper 
sight, more delicate hearing or scent, than the bulk of its 
fellows, must plainly have a great advantage in the struggle 
for existence. It will be sooner warned of the approach of 
an enemy ; it will more readily deted the presence of its 
prey, and will escape a number of subtle dangers to which 
it might otherwise succumb. Thus most of our readers 
