Chap. V.] LAWS OF VAKIATION. 165 



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 two factors, the nature of the organism, winch is 

 much the most important of the two, and the nature 

 of the conditions. The direct action of changed con- 

 ditions 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 subjected 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 con- 

 ditions, such as of 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- 



