Chap. XXI. FAVOURABLE CIRCUMSTANCES. 219 



formerly shown, no variety is quite uniform in character. 

 The same thing, as I am assured by nurserymen, would take 

 place in our flower-gardens, if the seed of the different 

 varieties were not separately saved. When the eggs of the 

 wild and tame duck are hatched together, the young wild 

 ducks almost invariably perish, from being of smaller size 

 and not getting their fair share of food. 39 



Facts in sufficient number have now been given showing 

 that natural selection often checks, but occasionally favours, 

 man's power of selection. These facts teach us, in addition, 

 a valuable lesson, namely, that we ought to be extremely 

 cautious in judging what characters are of importance in a 

 state of nature to animals and plants, which have to struggle 

 for existence from the hour of their birth to that of their 

 death, — their existence depending on conditions, about which 

 we are profoundly ignorant. 



Circumstances favourable to Selection by Man. 



The possibility of selection rests on variability, and this, as 

 we shall see in the following chapters, mainly depends on 

 changed conditions of life, but is governed by infinitely com- 

 plex and unknown laws. Domestication, even when long 

 continued, occasionally causes but a small amount of varia- 

 bility, as in the case of the goose and turkey. The slight 

 differences, however, which characterise each individual 

 animal and plant would in most, probably in all, cases suffice 

 for the production of distinct races through careful and pro- 

 longed selection. We see what selection, though acting on 

 mere individual differences, can effect when families of cattle, 

 sheep, pigeons, &c, of the same race, have been separately 

 bred during a number of years by different men without any 

 wish on their part to modify the breed. We see the same 

 fact in the difference between hounds bred for hunting in 

 different districts, 40 and in many other such cases. 



In order that selection should produce any result, it is 

 manifest that the crossing of distinct races must be prevented ; 

 hence facility in pairing, as with the pigeon, is highly 



39 Mr. Hewitt and others, in ' Jour- 40 ' Encyclop. of Rural Sports,' p. 



nal of Hort.,' 1862. p. 773. 405. 



