Chap. XXVIII. CONCLUDING REMARKS. 405 



It has sometimes been said that our domestic races do not 

 differ in constitutional peculiarities, but this cannot be main- 

 tained. In our improved cattle, pigs, &c., the period of 

 maturit}', including that of the second dentition, has been 

 much hastened. The period of gestation varies much, and 

 has been modified in a fixed manner in one or two cases. In 

 some breeds of poultry and pigeons the period at which the 

 down and the first plumage are acquired, differs. The number 

 of moults through which the larvae of silk-moths j)ass, varies. 

 The tendency to fatten, to yield much milk, to produce many 

 young or eggs at a birth or during life, differs in different 

 breeds. We find different degrees of adaptation to climate, 

 and different tendencies to certain diseases, to the attacks of 

 parasites, and to the action of certain vegetable poisons. 

 AVith plants, adaptation to certain soils, the power of resisting 

 frost, the period of flowering and fruiting, the duration of 

 life, the period of shedding the leaves or of retaining them 

 throughout the •winter, the proportion and nature of certain 

 chemical compounds in the tissues or seeds, all vary. 



There is, however, one important constitutional difference 

 between domestic races and species ; I refer to the sterility 

 which almost invariably follows, in a greater or less degree, 

 when species are crossed, and to the perfect fertility of the 

 most distinct domestic races, with the exception of a very 

 few plants, when similarly crossed. It is certainly a most 

 remarkable fact that many closely-allied species, which in 

 appearance differ extremely little, should yield when crossed 

 only a few more or less sterile offspring, or none at all ; 

 whilst domestic races which differ conspicuously from each 

 other are, when united, remarkably fertile, and yield perfectly 

 fertile offspring. But this fact is not in reality so inexplicable 

 as it at first appears. In the first place, it was clearly shown 

 in the nijieteenth chapter that the sterility of crossed species 

 does not depend chiefly on differences in their external struc- 

 ture or general constitution, but on differences in the repro- 

 ductive system, analogous to those which cause the lessened 

 fertility of the illegitimate unions of dimorphic and trimorphic 

 plants. In the second place, the Pallasian doctrine, that 

 species after having been long domesticated lose their natural 



