66 EVOLUTION OF LIVING OKGANISMS. 



necessarily adapted viability is the first test they have 

 to pass. Any that fail to satisfy it are eliminated at once. 

 But organisms in competition are further tested in par- 

 ticular directions, and when we can see both the advan- 

 tage gained and the means whereby it is obtained, the 

 special structure evolved, we speak of it in teleological 

 language as adapted for this purpose. Thus a sense 

 organ becomes specially adapted to receive only certain 

 stimuli ; a flower is adapted to be fertilised by insects. 

 Now each step in the formation of such a structure, each 

 fortuitous favourable variation selected to build it up, 

 may, in a sense, be said to have been preadapted. But 

 the word adaptation also implies just that process of 

 gradual building, that continued selection of variations 

 in a particular direction from among variations in all 

 other possible directions, which is represented in the 

 elaborated organ, yet cannot in any intelligible sense be 

 said to be present in each individual step. 



To prove that natural selection is a real working 

 principle, it must be shown that the death-rate is selective. 

 Unfavourable conditions, diseases, enemies of all kinds, 

 dog the path of every individual from birth to death ; 

 the death-rate is consequently enormous, as was shown 

 above. But is it really selective ? Are, for instance, the 

 two flies which alone survive out of every 20,000 (p. 61) 

 different on the average from those 19,998 which perish ? 

 That is the important point now to be considered. 



Doubtless in many cases the death of organisms is due 

 to pure "accident," causes which exterminate without 

 selective action, as for instance in the wholesale destruc- 

 tion brought about by some natural cataclysm ; but such 

 eases must be comparatively rare, and may be neglected. 

 Death of this kind simply fails to select; it does not 

 prevent: it merely delays the work of selection. The 

 survival of the fittest, the elimination of the unfit, is easy 

 enough to prove in the case of well-defined races, species, 

 and larger groups, and has been eloquently described by 



