Chap. v. Laws of Variation. 107 



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 coloured 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 atmos- 

 phere, 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 else- 

 where fleshy. These slightly varying organisms are interesting in 

 as far as they present characters analogous 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 further north they live ; but who can tell 

 how much of this difference may be due to the warmest-clad indivi- 

 duals having been favoured 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 domes- 

 tic 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. 

 Agam, 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 con- 

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

 quite ignorant. 



vJ^-? 61180 ^ 6 conditions of life may be said, not only to cause 

 rXiTS' *! ther r direc % ^ indirectly, but likewise to include 

 vn o?v I n ; f ° r thG condition s determine whether this or that 



cClTseetn^w ' ^ When man is the selectin S ^ ™ 

 clearly see that the two elements of change are distinct ; variability 



