LAWS OF VARIATION. 137 



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 conditions, 

 such as of climate, food, etc., 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 follow- 

 ing cases the conditions seem to have produced some slight 

 definite effect: B. Forbes asserts that shells at their 

 southern limit, and when living in shallow water, are more 

 brightly colored 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 colored under a clear 

 atmosphere, than when living near the coast or on islands; 

 and Wollaston is convinced that residence near the sea 

 affects the colors 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 pos- 

 sessed by the species which are confined to similar condi- 

 tions. 



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 individuals having been favored and preserved during 

 many generations, 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- 

 duced from the same species under external conditions 

 of life as different as can well be conceived; and, on the 



