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410 CONCLUDING REMARKS. Chap. XXVIII. 



our poultry and pigeons the acquirement of down and of the 

 first plumage by the young, and of the secondary sexual cha- 

 racters by the males, differ. The number of moults thron 

 which the larvae of silk-moths pass, varies. The tendency 

 fatten, to yield much milk, to produce many young or egcs 

 at a birth or during life, differs in different breeds. We find 

 different degrees of adaptation to climate, and different ten- 

 dencies to certain diseases, to the attacks of parasites, and to 

 the action of certain vegetable poisons. With plants, adaptation 



with some kinds of plums, the power of 



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

 the period of shedding the leaves and of retaining them through- 

 out 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 dis- 

 tinct domestic races, with the exception of a very few plants, 

 when similarly crossed. It certainly appears a remarkable fact 

 that many closely allied species which in appearance differ ex- 

 tremely little should yield when united 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 fer- 

 tile, 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 nineteenth chapter that the sterility 

 of crossed species does not closely depend on differences in their 

 external structure or general constitution, but results exclusively 

 from differences in the reproductive system, analogous with those 

 which cause the lessened fertility of the illegitimate unions and 

 illegitimate offspring of dimorphic and trimorphic plants. In 

 the second place, the Pallasian doctrine, that species after having 

 been long domesticated lose their natural tendency to sterility 

 when crossed, has been shown to be highly probable ; we can 

 scarcely avoid this conclusion when we reflect on the parentage 

 and present fertility of the several breeds of the dog, of Indian 

 and European cattle, sheep, and pigs. Hence it would be un- 

 reasonable to expect that races formed under domestication 





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