190 A Physicist on Evolution. 



convey; and tliat is the impression of the vast amount of labour, 

 both of observation and of thought, imphed in its production. Let 

 us glance at its principles. 



It is conceded on all hands that what are called varieties are 

 continually produced. The rule is probably without exception. 

 No chick and no child is in all respects and particulars the coun- 

 terpart of its brother or sister; and in such differences we have 

 " variety" incipient. No naturalist could tell how far this variation 

 could be carried ; but the great mass of them held that never by 

 any amount of internal or external change, nor by the mixture of 

 both, could the offspring of the same progenitor so far deviate from 

 each other as to constitute different species. The function of the 

 experimental philosopher is to combine the conditions of nature and 

 to produce her results ; and this was the method of Dtirwin.* He 

 made himself acquainted with what could, without any matter of 

 doubt, be done in the way of producing variation. He associated 

 himself with pigeon-fanciers — bought, begged, kept, and observed 

 every breed that he could obtain. Though derived from a common 

 stock, the diversities of these pigeons were such that " a score of 

 them might be chosen which, if shown to an ornithologist, and he 

 were told that they were wild birds, would certainly be ranked by 

 him as well-defined species." The simple principle which guides 

 the pigeon-fancier, as it does the cattle-breeder, is the selection of 

 some variety that strikes his fancy, and the propagation of this 

 variety by inheritance. With his eye still upon the particular 

 appearance which he wishes to exaggerate, he selects it as it 

 reappears in successive broods, and thus adds increment to incre- 

 ment until an astonishing amount of divergence from the parent 

 type is effected. Man in this case does not produce the elements of 

 the variation. He simply observes them, and by selection adds 

 them together until the required result has been obtained. " No 

 man," says Mr. Darwin, " would ever try to make a fantail till he 

 saw a pigeon with a tail developed in some slight degree in an 

 unusual manner, or a pouter until he saw a pigeon with a crop of 

 unusual size." Thus nature gives the hint, man acts upon it, and 

 by the law of inlieritance exaggerates the deviation. 



Having thus satisfied himself by indubitable facts that the 

 organization of an animal or of a plant (for pirecisely the same 

 treatment applies to j)lants) is to some extent plastic, he passes 

 from variation under domestication to variation under nature. 

 Hitherto we have dealt with the adding together of small changes 

 by the conscious selection of man. Can Nature thus select? 

 Mr. Darwin's answer is, "Assuredly she can." The number of 



* The first step only towards experimental demon stration Ims lieen taken. 

 Experiments now begun might, a couple of centuries hence, furnish data of incal- 

 culable value, which ought to bo supplied to the science of the future. 



