LAWS OF VARIATION 109 



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 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 liv- 

 ing 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 

 Woliaston 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 or- 

 ganisms 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 can- 

 not tell how much to attribute to the accumulative action of nat- 

 ural selection, and how much to the definite action of the condi- 

 tions 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 pre- 

 served 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 produced 

 from the same species under external conditions of life as different 

 as can well 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 conditions, than on a tendency to vary, due to 

 causes of which we are quite ignorant. 



In one sense the conditions of life may be said, not only to 

 cause variability, either directly or indirectly, but likewise to in- 

 clude natural selection, for the conditions determine whether this 



