The Progress of the 
[October, 
456 
speed and strength isolated in this respeCt. Something very 
similar will prevail concerning the advantage which animals 
gain by their so-called “ protective ” resemblances, either to 
other species or to their inanimate surroundings. Let us 
suppose a creature ill-adapted to escape from its enemies 
by speed or strength ; conspicuous in its form and colour- 
ation, and therefore unable to conceal itself ; and, lastly, 
attractive to the smell and taste of rapacious animals, and 
consequently eagerly sought for by them as food. If, now, 
one individual of the species varies in colour from the 
normal standard in a direction slightly verging towards a 
protective hue, the advantage that it will hence derive in 
the struggle for existence will be equally trifling, even al- 
though a multiplicity of steps such as it has just taken 
might finally render the modified form scarcely perceptible 
to its enemies. Or we may suppose that one individual of 
the persecuted species takes the first step towards the deve- 
lopment of a repulsive odour. Here, also, its chances of exist- 
ence will not be perceptibly increased, though its devourer, if 
able to reflect so far, may perhaps think that the morsel was 
not quite so good as usual. 
We submit, therefore, that under a multitude of circum- 
stances, if variations of colour or odour, or augmentations 
of speed, are to give the individual thus modified a greater 
chance of survival, they must either occur simultaneously 
in a considerable number of specimens, or they must be 
advances in the required direction, not slight and scarcely 
perceptible, but well-marked. 
There is another and a different consideration which in 
our opinion must not be overlooked, as powerfully tending 
to modify the influence of Natural Selection. It has been 
argued that individuals favourably modified in any way, but 
especially as regards strength or swiftness, will stand a much 
better chance, not merely of escaping their enemies or se- 
curing their prey, but also of obtaining mates and leaving 
offspring. Yet, so far as birds are concerned, this advan- 
tage, be it great or small, appears to be neutralised. In 
Mr. Wallace’s work we find the following passage: — 
“ Again, the evidence collected by Mr. Darwin himself 
proves that each bird finds a mate under any circumstances. 
He gives a number of cases of one of a pair of birds being 
shot, and the survivor being always found paired again 
almost immediately. This is sufficiently explained on the 
assumption that the destruction of birds by various causes 
is continually leaving widows and widowers in nearly equal 
proportions, and thus each one finds a fresh mate, and it 
