146 ORIGIN OF SPECIES 



sation seems to become plastic, and we have much fluctu- 

 ating 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 be- 

 come modified in the same way. 



It is very difficult to decide how far changed conditions, 

 such as of climate, food, &c., have acted in a definite man- 

 ner. 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 innumer- 

 able 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 beUeves 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 analogous to those possessed by the spe- 

 cies 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 species have thicker and better 

 fur the further north they live; but who can tell how much 

 of this difference may be due to the warmest-clad individu- 

 als having been favoured and preserved during many genera- 

 tions, and how much to the action of the severe climate? 

 for it would appear that climate has some direct action on 

 the hair of our domestic quadrupeds. 



Instances could be given of similar varieties being pro- 



