en. xii.] ADJUSTMENT, DIRECT AND INDIRECT. 56 



taneously vary in a given direction, and if this may often 

 happen within the limits of single generations, it is obvious 

 that we have here a factor of specific change not to be lightly 

 passed over. In estimating the effects of natural selection 

 upon a number of variations which are, quite legitimately, 

 taken for granted, we must not forget to generalize the varia- 

 tions in connection with some common cause to which they 

 may be assignable. Now it cannot be denied that in any 

 single generation of organisms variations are very likely to 

 occur, throughout nearly the whole number of individuals, 

 which are due to the direct adaptation of the species to its 

 environing circumstances. When exhibited in the effects 

 wrought upon the human constitution by exposure to changed 

 physical conditions, such variations are known as acclimatiza- 

 tion. Within the infinitesimal period of two centuries the 

 English race in America has come to differ perceptibly, though 

 very slightly, from the English race in Europe ; and this \ ery 

 slight difference, which cannot be explained by the much 

 overrated hypothesis of the infusion of foreign blood, and 

 which certainly cannot be traced to natural selection, must 

 be almost wholly due to direct adaptation to new physical 

 and social conditions. Of kindred import is the fact that 

 "twenty-nine kinds of American trees all differ from their 

 nearest European allies in a similar manner, having leaves 

 less toothed, buds and seeds smaller, fewer branchlets, etc," 

 So ]\I. Costa states " that young shells taken from the shores 

 01 England and placed in the Mediterranean at once altered 

 their manner of growth, and formed prominent diverging rays 

 like those on the shells of the proper Mediterranean oyster." 

 We have seen that the direct action of physical agencies will 

 by no means account for the chief features of colouring in 

 the organic world ; yet it appears to be true that members of 

 the same species of birds are more brightly coloured when 

 living in a clear dry atmosphere than when living near the 

 coast. So, too, in the contour of their wings, the various 



