Chap. V]. LAWS OF VAfilATION. 165 



first chapter I attempted to show that changed con- 

 ditions act in two ways, directly on the whole organi- 

 sation or on certain parts alone, and indirectly through 

 the reproductive system. In all cases there are two 

 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 variahility. 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 col- 

 oured 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 WoUaston is convinced that residence 

 near the sea affects the colours of insects. Moquin- 



