XVII THE METHOD OF ORGANIC EVOLUTION 349 



of the extreme from the mean value of any organ or part 

 is usually from one-tenth to one-fourth, sometimes as 

 great as one-third of the mean value, with usually a perfect 

 gradation of intervening values. 



The multiplication of individuals of all species is so 

 great and so rapid that only a small proportion of those 

 born each year can possibly survive ; hence a struggle for 

 existence necessarily follows, the result of which is that, 

 on the average, those individuals which are in any way 

 ill-fitted for the conditions of existence die, while those 

 better fitted live. The struggle is of varied character 

 and intensity — either with the forces of nature, as cold, 

 drought, storms, floods, snow, &c. ; with other creatures, 

 in order to escape being devoured, or to obtain food, 

 whether for themselves or for their offspring; or with 

 their own race in the competition for mates and for the 

 means of existence ; while as regards all these forms of 

 struggle mental and social qualities are often as im- 

 portant as mere physical perfection, and sometimes much 

 more important. The fact already stated, of the large 

 amount of variability in most species, has been thought 

 by some to show either that there can be no such severe 

 struggle as has been suggested, or that the characters 

 which vary so much can be of little importance to the 

 species, and cannot therefore determine survival. But in 

 making this objection two considerations have been over- 

 looked. In the first place we always compare adults, and 

 an enormous amount of destruction has already taken 

 place during the earlier stages of life. The adults, there- 

 fore, are already a selected group. In the second place, 

 the struggle is very largely intermittent, owing both to 

 the occurrence only at long intervals of the most adverse 

 meteorological conditions, while the diversity of these 

 conditions leads in each case to the selection of a different 

 characteristic. An exceptionally severe winter will de- 

 stroy all which are deficient in one set of characters, 

 while a long drought, or scarcity of some particular kind 

 of food, will weed out those deficient in another set of 

 characters. Thus, in any one year there will exist 

 nuQibers of individuals which are doomed to speedy de- 



