Chap. XXI. FAVOURABLE CIRCUMSTANCES. 233 



are less crowded together. The same observer maintains that 

 in the upright varieties the divergent awns are serviceable by- 

 breaking the shocks when the ears are dashed together by the 

 wind. 36 If several varieties of a plant are grown together, and 

 the seed is indiscriminately harvested, it is clear that the 

 hardier and more productive kinds will, by a sort of natural 

 selection, gradually prevail over the others ; this takes place, as 

 Colonel Le Couteur believes, 37 in our wheat-fields, for, as for- 

 merly 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 inva- 

 riably perish, from being of smaller size and not getting their 

 fair share of food. 38 



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 from the hour of 

 their birth to that of their death for existence, — 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 complex, 

 and, to a great extent, unknown laws. Domestication, even 

 when long continued, occasionally causes but a small amount 

 of variability, 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, 



3G ' Gardener's Chronicle,' 1862, pp. 820, 821. 



# ' On the Varieties of Wheat,' p. 59. 



38 Mr. Hewitt and others, in ' Journal of Hort.,' 1862, p. 773. 



