14 STERILITY 



respiration, their pulse, their instinct, which are all 

 suddenly modified, can it be wondered at that they 

 are incapable of breeding? I think it may be truly 

 said it would be more wonderful if they did. But it 

 may be asked why have not the recognised varieties, 

 supposed to have been produced through the means 

 of man, [not refused to breed] have all bred 1 . 

 Variation depends on change of condition and 

 selection 2 , as far as man's systematic or unsystematic 

 selection (has) gone; he takes external form, has 

 little power from ignorance over internal invisible 

 constitutional differences. Races which have long 

 been domesticated, and have much varied, are 

 precisely those which were capable of bearing great 

 changes, whose constitutions were adapted to a 

 diversity of climates. Nature changes slowly and 

 by degrees. According to many authors probably 

 breeds of dogs are another case of modified species 

 freely crossing. There is no variety which (illegible) 

 has been (illegible) adapted to peculiar soil or 

 situation for a thousand years and another rigor- 

 ously adapted to another, till such can be produced, 

 the question is not tried 3 . Man in past ages, could 

 transport into different climates, animals and plants 

 which would freely propagate in such new climates. 

 Nature could effect, with selection, such changes 

 slowly, so that precisely those animals which are 

 adapted to submit to great changes have given rise to 

 diverse races, and indeed great doubt on this head 4 . 



1 This sentence ends in confusion : it should clearly close with the words 

 "refused to breed" in place of the bracket and the present concluding 

 phrase. 



2 The author doubtless refers to the change produced by the summation 

 of variation by means of selection. 



3 The meaning of this sentence is made clear by a passage in the MS. of 

 1844: "Until man selects two varieties from the same stock, adapted to 

 two climates or to other different external conditions, and confines each 

 rigidly for one or several thousand years to such conditions, always selecting 

 the individuals best adapted to them, he cannot be said to have even 

 commenced the experiment." That is, the attempt to produce mutually 

 sterile domestic breeds. 



4 This passage is to some extent a repetition of a previous one and may 



