Chap. XIX. 



FOUK LAST CHAPTERS. 



177 



fruits, and in some cases greatly developed tubers, &c, to 

 incipient sterility of the above nature combined with a copious 

 supply of nutriment. Animals which have long been domes- 

 ticated, and plants which have long been cultivated, can gene- 

 rally withstand, with unimpaired fertility, great changes in their 

 conditions of life ; though both are sometimes slightly affected. 

 With animals the somewhat rare capacity of breeding freely 

 under confinement has mainly determined, together with their 

 utility, the kinds which have been domesticated. 



We can in no case precisely say what is the cause of the 

 diminished fertility of an animal when first captured, or of a 

 plant when first cultivated ; we can only infer that it is caused 

 by a change of some kind in the natural conditions of life. The 

 remarkable susceptibility of the reproductive system to such 

 changes,— a susceptibility not common to any other organ,— 

 apparently has an important bearing on Variability, as we shall 

 see in a future chapter. 



It is impossible not to be struck with the double parallelism 

 between the two classes of facts just alluded to. On the one 

 hand, slight changes in the conditions of life, and crosses 

 between slightly modified forms or varieties, are beneficial as far 

 as prolificness and constitutional vigour are concerned. On the 

 other hand, changes in the conditions greater in degree, or of a 

 different nature, and crosses between forms which have been 

 slowly and greatly modified by natural means— in other words, 

 between species— are highly injurious, as far as the reproductive 

 system is concerned, and in some few instances as far as consti- 

 tutional vigour is concerned. Can this parallelism be accidental ? 

 Does it not rather indicate some real bond of connection ? As 

 a fire goes out unless it be stirred up, so the vital forces are 

 always tending, according to Mr. Herbert Spencer, to a state of 

 equilibrium, unless disturbed and renovated through the action 

 01 other forces. 



In some few cases varieties tend to keep distinct, by breeding 

 at different periods, by great differences in size, or by sexual 

 preference,— m this latter respect more especially resembling 

 species m a state of nature. But the actual crossing of 

 varieties, far from diminishing, generally adds to the fertility of 

 t union and the mongrel offspring. Whether all 



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VOL. II. 



