CHAP. V.] LAWS OF VARIATION. 99 



ledge plainly our ignorance of the cause of each particular varia- 

 tion. Some authors believe it to be as much the function of the 

 reproductive system to produce individual differences, or slight 

 deviations of structure, as to make the child like its parents. But 

 the fact of variations and monstrosities occurring much more fre- 

 quently under domestication than under nature, and the greater 

 variability of species having wide ranges than of those with 

 restricted ranges, lead to the conclusion that variability is generally 

 related to the conditions of life to which each species has been 

 exposed during several successive generations. In the first chapter ' 

 I attempted to show that changed conditions act in two ways, 

 directly on the whole organisation or on certain parts alone, and 

 indirectly through the reproductive system. In all cases there are 

 fATC factors, the nature of the organism, which is much the most 

 important of the two, and the nature of the conditions. The direct 

 action of changed conditions leads to definite or indefinite results. 

 In the latter case the organisation seems to become plastic, and 

 we have much fluctuating variability. In the former case the 

 nature of the organism is such that it yields readily, when sub- 

 jected to certain conditions, and all, or nearly all the individuals 

 become modified in the same way. 



It is very difficult to decide how far changed conditions, such as 

 f climate, food, &c., have acted in a definite manner. There is 

 reason to believe that in the course of time the effects have been 

 greater than can be proved by clear evidence. But we may safely 

 conclude that the innumerable complex co-adaptations of structure, 

 which we see throughout nature between various organic beings, 

 cannot be attributed simply to such action. In the following 

 cases the conditions seem to have produced some slight definite 

 effect : E. Forbes asserts that shells at their southern limit, and 

 when living in shallow water, are more brightly coloured than 

 those of the same species from further north or from a greater 

 depth ; but this certainly does not always hold good. Mr. Gould 

 believes that birds of the same species are more brightly coloured 

 under a clear atmosphere, than when living near the coast or on 

 islands; and Wollaston is convinced that residence near the sea 

 affects the colours of insects. Moquin-Tandon gives a list of plants 

 which, when growing near the sea-shore, have their leaves in some 

 degree fleshy, though not elsewhere fleshy. These slightly varying 

 organisms are interesting in as far as they present characters analo- 

 gous to those possessed by the species which are confined to similar 

 conditions. 



"When a variation is of the slightest use to any being, we cannot \ 

 tell how much to attribute to the accumulative action of natural 

 selection, and how much to the definite action of the conditions of / 

 life. Thus, it is well known to furriers that animals of the same 



