174 HYBRIDISM. Chap. XIX. 



that tlie species, owing to their struggle for existence with 

 numerous competitors, must have been exposed to more uni- 

 form conditions of life during long periods of time than 

 domestic varieties have been, and this may well make a 

 wide diiference in the result. For we know how commonly 

 wild animals and plants, when taken from their natural 

 conditions and subjected to captivity, are rendered sterile ; 

 and the reproductive functions of organic beings which have 

 always lived and been slowly modified under natural con 

 ditions would probably in like manner be eminently sensitive 

 to the influence of an unnatural cross. Domesticated pro- 

 ductions, on the other hand, which, as shown by the mere fact 

 of their domestication, were not originally highly sensitive 

 to changes in their conditions of life, and which can now 

 generally resist with undiminished fertility repeated changes 

 of conditions, might be expected to produce varieties, which 

 would be little liable to have their reproductive powers inju- 

 riously affected by the act of crossing with other varieties 

 which had originated in a like manner. 



Certain naturalists have recently laid too great stress, as it 

 appears to me, on the difference in fertility between varieties 

 and species when crossed. Some allied species of trees cannot 

 be grafted on one another, whilst all varieties can be so 

 grafted. Some allied animals are affected in a very different 

 manner by the same poison, but with varieties no such case 

 until recently was known ; whilst now it has been proved that 

 immunity from certain poisons sometimes stands in correla- 

 tion with the colour of the individuals of the same species. 

 The period of gestation generally differs much in distinct 

 sx)ecies, but with varieties until lately no such difference had 

 been observed. Here we have various phj^siological differences, 

 and no doubt others could be added, between one species and 

 another of the same genus, which do not occur, or occur with 

 extreme rarity, in the case of varieties ; and these differences 

 are apparently wholly or in chief part incidental on other 

 constitutional differences, just in the same manner as the 

 sterility of crossed species is incidental on differences confined 

 to the sexual sj^stem. Why, then, should these latter differ- 

 ences, however serviceable they may indirectly be in keej)ing 



