PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 395 



action of the severe climate? For it would appear that climate has 

 some direct action on the hair of our domestic quadrupeds."" Since, 

 however, it happens that some species do not vary at all, although 

 living under the most opposite climates, he is thereby inclined " not 

 to lay much weight on the direct and definite action of the conditions 

 of life," though he fully admits " that strong arguments of a general 

 natui-e may be advanced on the other side." " In one sense," he adds, 

 " the conditions of life may be said not only to cause variability, but 

 likewise to include natural selection, for the conditions determine 

 Avhether this or that variety shall survive." ^ But he says again: " I 

 believe that natural selection generally acts slowdy in effecting 

 changes at long hitcrrah of time mid only on a few of the inhabitants 

 of the same region.'''' In a later work, however, he refers to the vari- 

 ation in color with locality seen in many species of birds in the 

 United St^ites, and says explicitly, in reference to northern and 

 southern localities, " this seems to be the direct result of the difference 

 in temperature, light, etc., between the two regions." '^ 



There is, however, a A^ast amount of unquestionable proof of the 

 direct and constant action of climate and other conditions of life 

 upon animals, and that such geographical variations as the thicker 

 and softer fur of mammals inhabiting cold regions, smaller size and 

 bi-ighter colors at the soutliAvard, etc., do not require the action 

 of natural selection, in its strict and proper sense, for their explana- 

 tion. It is well known, for instance, that a flock of fine-wooled sheep 

 when taken to a hot climate rapidly acquire a coarser and coarser 

 fleece, till, in a few generations, it nearly loses its character of proper 

 wool, and becomes simply hair; that the change affects simultaneously 

 the whole flock, and is not brought about by one or two individuals 

 acquiring a coarser fleece and through their descendants modifying 

 the character of the herd. Furthermore, in the case of sheep, it is 

 well known that certain countries are very favorable to the production 

 of a fine fleece, and that fine-wooled breeds, even by man's aid, can 

 not be perpetuated in other regions. Again, it is a fact of common 

 observation that in birds and mammals colors become more or less 

 faded toward the moulting season simply by the direct action of the 

 elements, the tints of the fresh and the long-worn plumage or 

 pelage being more or less strikingly different in the same individ- 

 uals, and that this contrast at different seasons is more marked in 

 arid than in moist regions, through the greater bleaching effect of a 

 dry heated atmosphere and the more intense dazzling sunlight of 

 regions that are not only cloudless, but lack the protection afforded 

 hy abundant vegetation. 



<' Orisin of Spwies. pp. 166, 167. "The Descent of Man, 2a. ed., p. 22.5 



^ Origin of Species, p. 168. 



