120 LAWS OF VARIATION. 



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 innu- 

 merable complex coadaptations 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 colored 

 than those of the same species from farther 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 con- 

 vinced 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 organ- 

 isms 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 species have thicker and better fur 

 the farther 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 w^ell be conceived; and, on the other 

 hand, of dissimilar varieties being produced under apparently 

 the same external conditions. Again, innumerable instances 

 are known to every naturalist, of species keeping true, or 

 not varying at all, although living under the most opposite 

 climates. Such considerations as these incline me to lay 

 less weight on the direct action of the surrounding condi- 

 tions, than on a tendency to vary, due to causes of which we 

 are quite ignorant. 



