Chap. XXVIII. CONCLUDING EEMAEKS. 419 



hermaphrodite plants we incessantly meet with elaborate and 

 perfect contrivances for this same end. It is no exagge- 

 ration to assert that, if the use of the talons and tusks of a 

 carnivorous animal, or of the plumes and hooks on a seed, may 

 be safely inferred from their structure, we may with equal 

 safety infer that many flowers are constructed for the express 

 purpose of ensuring a cross with a distinct plant. From 

 these various considerations, not to mention the result of a 

 long series of experiments which I have tried, the conclusion 

 arrived at in the chapter just referred to — namely, that great 

 good of some kind is derived from the sexual concourse of 

 distinct individuals — must be admitted. 



To return to our illustration : we have hitherto assumed 

 that the birds were kept down to the same number by indis- 

 criminate slaughter ; but if the least choice be permitted in 

 their preservation, the whole result will be changed. Should 

 the owner observe any slight variation in one of his birds, 

 and wish to obtain a breed thus characterised, he would succeed 

 in a surprisingly short time by careful selection. As any 

 part which has once varied generally goes on varying in the 

 same direction, it is easy, by continually preserving the most 

 strongly marked individuals, to increase the amount of differ- 

 ence up to a high, predetermined standard of excellence. 

 This is methodical selection. 



If the owner of the aviary, without any thought of making 

 a new breed, simply admired, for instance, short-beaked more 

 than long-beaked birds, he would, when he had to reduce the 

 number, generally kill the latter ; and there can be no doubt 

 that he would thus in the course of time sensibly modify his 

 stock. It is improbable, if two men were to keep pigeons 

 and act in this manner, that they would prefer exactly the 

 same characters ; they would, as we know, often prefer 

 directly opposite characters, and the two lots would ulti- 

 mately come to differ. This has actually occurred with 

 strains or families of cattle, sheep, and pigeons, which have 

 been long kept and carefully attended to by different breeders, 

 without any wish on their part to form new and distinct sub- 

 breeds. This unconscious kind of selection will more espe- 

 cially come into action with animals which are highly scrvice- 



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