68 EVOLUTION OF LIVING ORGANISMS. 



swamp. The survival of some forms, the extinction of 

 others less well adapted, the alternate spread of one 

 species and retreat of another as the climatic conditions 

 oscillate from dryness to dampness, from heat to cold, 

 these are familiar to every botanist. 



We need not dwell longer on this aspect of the question. 

 These and similar facts are not ignored by the most 

 sceptical of the opponents of Darwinism ; nor is their 

 significance denied. But they will say "Granted that 

 the great gaps between the widely divergent forms have 

 been produced by the extinction of the intermediate ones, 

 granted that the struggle between differing species leads 

 to the selection of the best adapted, these well-marked 

 * species' were already present. Show us," they will 

 ask, " selection between varieties differing but little from 

 each other, and between the individual variations them- 

 selves." Instances of the success of mere " varieties " 

 are not unknown. Of late years it has been observed that 

 dark varieties of various moths have tended to increase 

 in northern England, and may even have superseded the 

 original pale forms, as in the case of the peppered moth 

 (Amphidasys betularia). Familiar to us all, though per- 

 haps rarely understood, are the constant struggles be- 

 tween the various races of mankind, which have played 

 and still play so large a part in the history of the world ; 

 here there is ample material for the study of the selective 

 value of all sorts of racial characters. More difficult is it 

 to demonstrate the action of selection among individuals, 

 and to express the result in statistical form. Only on 

 rare occasions can we directly compare the eliminated 

 individuals with the survivors. One good example we 

 owe to the American zoologist Bum pus. After a severe 

 storm he collected 136 injured specimens of the common 

 sparrow (Passer domesticus) ; and out of this number 

 72 revived, while 64 did not recover. On measuring all 

 the birds, and comparing the dead with the survivors, 

 k was found that the former on the average were 



