Wallace • The Debt of Science to Darwin 



273 



STUDIES OF DOMESTIC ANIMALS 



Although, as we have said, Darwin 

 had early arrived at the conclusion that 

 allied species had descended from 

 common ancestors by gradual modifi- 

 cation, it long remained to him an 

 inexplicable problem how the neces- 

 sary degree of modification could have 

 been effected and he adds: "It would 

 thus have remained forever, had I not 

 studied domestic productions and thus 

 acquired a just idea of the power of 

 selection." These researches were pub- 

 lished at length in two large volumes, 

 with the title Animals and Plants under 

 Domestication. In order to determine 

 the nature and amount of the variabil- 

 ity of domestic productions, he pre- 

 pared skeletons of all the more impor- 

 tant breeds of rabbits, pigeons, fowls, 

 and ducks, as well as of the wild races 

 from which thev are known to have 

 been produced. Another set of experi- 

 ments was made by crossing the dif- 

 ferent breeds of pigeons and fowls 

 which were most completely unlike 

 the wild race, with the result that in 

 many cases the offspring were more 

 like the wild ancestors than either of 

 the parents. These experiments, sup- 

 ported by a mass of facts observed by 

 other persons, served to establish the 

 principle of the tendency of crosses to 

 revert to the ancestral form, and this 

 principle enabled him to explain the 

 frequent appearance of stripes on 

 mules, and occasionally on dun-col- 

 oured horses, on the hypothesis that 

 the common ancestor of the horse, ass, 

 and zebra tribe was a partially striped 

 and dun-coloured animal. 



It was proved that the parts most 

 selected or which had alreadv most 

 varied were most subject to further 

 variation. Once a part had begun to 

 change, variations became more abun- 

 dant. It was found in many cases, when 

 much variation occurred, that there 

 was a tendency to a difference in the 



sexes which had not before existed. 

 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 to- 

 gether; the colour and size of the leaves 

 and seeds changing simultaneously in 

 some plants. 



The effect of disuse in causing the 

 diminution of an organ was exhibited. 

 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 fowl, the capacity 

 of the skull in tame rabbits, and the 

 size and strength of the wings in silk- 

 worm moths. 



Still more remarkable, perhaps, is 

 the collection of facts afforded by 

 plants. Notwithstanding the enormous 

 mass of facts and observations given, 

 the portion relating to plants is often 

 but an abstract of the results of his own 

 elaborate experiments, carried on for a 

 long series of years, and given at length 

 in three separate volumes on The Fer- 

 tilization of Orchids, On Cross and 

 Self -Fertilization of Plants, and on The 

 Forms of Flowers. These works may 

 be said to have revolutionized the sci- 

 ence of botany, since, for the first 

 time, they gave a clear and intelligible 

 reason for the existence of that won- 

 derful diversity in the form, colours, 

 and structures of flowers. The investi- 

 gation of the whole subject of crossing 

 and hybridity had shown that, although 

 hybrids between distinct species usu- 

 allv produced sterile offspring, yet 

 crosses between slightlv different vari- 

 eties led to increased fertility. A long 

 series of experimental researches es- 

 tablished the important proposition 

 that cross-fertilization is of the great- 

 est importance to the health, vigour, 

 and fertility of plants. In the case of 

 orchids, it was shown that those strange 

 and beautiful flowers owed their singu- 



