IV VARIATION UNDER DOMESTICATION 99 



Although I believe, for reasons which will be given further 

 on, that some amount of variability is a constant and 

 necessary property of all organisms, yet there appears to be 

 good evidence to show that changed conditions of life tend to 

 increase it, both by a direct action on the organisation and 

 by indirectly affecting the reproductive system. Hence the 

 extension of civilisation, by favouring domestication under 

 altered conditions, facilitates the process of modification. Yet 

 this change does not seem to be an essential condition, for 

 nowhere has the production of extreme varieties of plants and 

 flowers been carried farther than in Japan, where careful 

 selection continued for many generations must have been the 

 chief factor. The effect of occasional crosses often results in 

 a great amount of variation, but it also leads to instability of 

 character, and is therefore very little employed in the pro- 

 duction of fixed and well-marked races. For this purpose, in 

 fact, it has to be carefully avoided, as it is only by isolation and 

 pure breeding that any specially desired qualities can be in- 

 creased by selection. It is for this reason that among savage 

 peoples, whose animals run half w\\d, little improvement takes 

 place ; and the difficulty of isolation also ex|3lains why distinct 

 and pure breeds of cats are so rarely met Avith. The wide dis- 

 tribution of useful animals and plants from a very remote 

 epoch has, no doubt, been a powerful cause of modification, 

 because the particular breed first introduced into each country 

 has often been kept pure for many years, and has also been 

 subjected to slight differences of conditions. It ^Yil\ also 

 usually have been selected for a somewhat different purpose 

 in each locality, and thus very distinct races would soon 

 originate. 



The important physiological effects of crossing breeds or 

 strains, and the part this plays in the economy of nature, will 

 be explained in a future chapter. 



Concluding Remarks. 



The examples of variation now adduced — and these might 

 have been almost indefinitely increased — ^y\\\ suffice to show 

 that there is hardly an organ or a quality in plants or animals 

 which has not been observed to vary ; and further, that when- 

 ever any of these variations have been useful to man he has 



