Ix THE DEBT OF SCIENCE TO DARWIN 461 
this showed that, when once any part had begun to change, 
variations became more abundant, thus furnishing materials 
to render still further change in the same direction compara- 
tively easy. This is the secret of the rapid improvement of 
breeds or races, and is equally applicable to the formation of 
species by natural selection. Again, it was found that in 
many cases, when much variation occurred, there was a 
tendency to a difference in the sexes which had not before 
existed. This has been observed in sheep, in fowls, and in 
pigeons, and it is very interesting as indicating the origin of 
that wonderful diversity between the two sexes which occurs 
in several groups of animals. Another curious fact is the 
correlation of parts which occurs in many animals, such as 
the tusks and bristles of swine, and the hair and teeth in 
some dogs, both increasing or becoming lost together ; the 
beak and feet of pigeons, both increasing or diminishing 
together ; the colour and size of the leaves and seeds changing 
simultaneously in some plants ; and numerous other instances 
which serve to explain some of the peculiar characters of 
natural objects for which we can discover or imagine no 
direct use. 
The effect of disuse in causing the diminution of an organ 
was exhibited by careful comparison and measurements of 
tame and wild birds. The sternum, scapula, and furcula to 
which the muscles used in flight are attached, are found to be 
diminished in domestic pigeons, as were the wing-bones in 
domestic fowls, the capacity of the skull in tame rabbits, and 
the size and strength of the wings in silkworm moths. The 
evidence afforded by the breeds of pigeons (which have been 
domesticated for so many centuries and in so many parts of 
the world) of the process of selection, whether unconscious or 
methodical, is very clearly set forth, and serves as a typical 
example with which to compare the various phenomena pre- 
sented by allied species in a state of nature; and in con- 
cluding its discussion, he thus replies to some objections :— 
“T have heard it objected that the formation of the several 
domestic races of the pigeon throws no light on the origin of 
the wild species of the columbidz, because their differences 
are not of the same nature. The domestic races, for instance, 
do not differ, or hardly at all, in the relative lengths and 
