THE DEBT OF SCIENCE TO DARWIN 



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, scapulae, 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 : 



" I 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 columbidae, 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 



