£14 CONCLUDING REMARKS. Chap. XXYIIL 



indefinitely, causing diversified variations in nearly the 

 same manner as exposure to cold or the absorption of the 

 same poison affects different individuals in different ways. 

 We have reason to suspect that an habitual excess of highly- 

 nutritious food, or an excess relatively to the wear and tear 

 of the organisation from exercise, is a powerful exciting cause 

 of variability. When we see the symmetrical and complex 

 outgrowths, caused by a minute drop of the poison of a gall- 

 insect, we may believe that slight changes in the chemical 

 nature of the sap or blood would lead to extraordinary modi 

 fications of structure. 



The increased use of a muscle with its various attached 

 parts, and the increased activity of a gland or other organ, 

 lead to their increased development. Disuse has a contrary 

 effect. With domesticated productions, although their organs 

 sometimes become rudimentary through abortion, we have do 

 reason to suppose that this has ever followed solely from 

 disuse. With natural species, on the contrary, many organs 

 appear to have been rendered rudimentary through disuse, 

 aided by the principle of the economy of growth together 

 with intercrossing. Complete abortion can be accounted for 

 only by the hypothesis given in the last chapter, namely, the 

 final destruction of the germs or gemmules of useless parts. 

 This difference between species and domestic varieties may 

 be partly accounted for by disuse having acted on the 

 latter for an insufficient length of time, and partly from 

 their exemption from any severe struggle for existence 

 entailing rigid economy in the development of each part, to 

 which all species under nature are subjected. Nevertheless 

 the law of compensation or balancement, which likewise 

 depends on the economy of growth, apparently has affected 

 to a certain extent our domesticated productions. 



As almost every part of the organisation becomes highly 

 variable under domestication, and as variations are easily 

 selected both consciously and unconsciously, it is very diffi- 

 cult to distinguish between the effects of the selection of 

 indefinite variations and the direct action of the conditions 

 of life. For instance, it is possible that the feet of our wa ter- 

 es and of the American doo-s which have to travel much 



