Chap. XIX. FOUR LAST CHAPTERS. 161 



have been observed. We probably owe our double flowers, 

 rich seedless 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 domesticated, and plants which have long been 

 cultivated, can generally 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, together 

 with their utility, mainly determine 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 constitutional 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 of other 

 forces. 



In some few cases varieties tend to keep distinct, by breed- 

 ing at different seasons, by great difference in size, or by 

 sexual preference. But the crossing of varieties, far from 



