l.iS LAWS OF VARIATION. Chap. V. 



It is very difficult to decide how far clianii^cd conditions, 

 such as of climate, food, etc., have acted in a definite manner. 

 There is some reason to believe that in the course of time the 

 effects have been greater than can be proved to be the case by 

 any clear evidence. But we may safely conclude that the in- 

 niunerable complex coadaptations of structure which we see 

 throughout Nature between various organic beings, cannot be 

 attriljuted 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 these statements have lately been disputed, Mr. Gould 

 believes that birds of the same species are more brightly col- 

 ored under a clear atmosjihere, than when living on islands or 

 near the coast ; and WoUaston is convinced that residence near 

 tlie sea affects the colors of insects. Moquin-Tandon gives a 

 li.st of plants which when growing near the sea-shore have their 

 leaves in some degree ileshy, though not elsewhere fleshj'. 

 Other similar facts could be given. 



The fact of varieties of one species, when they range into 

 the habitations of other species, often acquiring in a slight 

 degree some of their characters, accords Avith the view that 

 species arc only well-marked and permanent varieties. Thus 

 the species of shells Avhich are confined to tropical and shal- 

 low seas are generally brighter-colored than those confined to 

 cold and deeper seas. The birds Avhich are confined to conti- 

 nents are, according to Mr. Gould, brighter-colored than those 

 of islands. The insect-species confined to sea-coasts, as every 

 collector knows, are often brassy or lurid. Plants which live 

 exclusively on the sea-side are very apt to have Ileshy leaves. 

 He who believes in the creation of each species, will have to 

 say that this insect, for instance, was created of a brassy color, 

 because it was intended to live near the sea, but that this other 

 insect became brassy through variation as soon as it reached 

 the sea-coast. 



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. Tlius, it is avcU known to furriers that ani- 

 mals of the same species have thicker and better fur tlie far- 

 tlier north they live ; but who can tell liow much of this dif- 

 ference may be due to the warmest-clad individuals having 



