THE FACTORS OF ORGANIC EVOLUTION. 765 



tions ? Considering how immense must be the number of these re- 

 quired changes, added to the changes above enumerated, the chances 

 against any adequate re-adjustments fortuitously arising must be in- 

 finity to one. 



If the effects of use and disuse of parts are inheritable, then any 

 change in the fore parts of the giraffe which affects the action of the 

 hind-limbs and back, will simultaneously cause, by the greater or less 

 exercise of it, a re-moulding of each component in the hind-limbs and 

 back in a way adapted to the new demands ; and generation after 

 generation the entire structure of the hind-quarters will be progress- 

 ively fitted to the changed structure of the fore-quarters : all the 

 appliances for nutrition and innervation being at the same time pro- 

 gressively fitted to both. But in the absence of this inheritance of 

 functionally-produced modifications, there is no seeing how the re- 

 quired re-adjustments can be made. 



Yet a third class of difficulties stands in the way of the belief that 

 the natural selection of useful variations is the sole factor of organic 

 evolution. This class of difficulties, already pointed out in § 166 of 

 the Principles of Biology, I cannot more clearly set forth than in the 

 words there used. Hence I may perhaps be excused for here quoting 

 them : 



" Where the life is comparatively simple, or where surrounding circum- 

 stances render some one function supremely important, the survival of the fittest 

 may readily bring about the appropriate structural change, without any aid from 

 the transmission of functionally-acquired modifications. But in proportion as 

 the life grows complex — in proportion as a healthy existence cannot be secured 

 by a large endowment of some one power, but demands many powers ; in the 

 same proportion do there arise obstacles to the increase of any particular power, 

 by " the preservation of favored races in the struggle for life." As fast as the 

 faculties are multiplied, so fast does it become possible for the several members 

 of a species to have various kinds of superiorities over one another. While one 

 saves its life by higher speed, another does the like by clearer vision, another by 

 keener scent, another by quicker liearing, another by greater strength, another 

 by unusual power of enduring cold or hunger, another by special sagacity, an- 

 other by special timidity, another by special courage ; and others by other bodily 

 and mental attributes. Now it is unquestionably true that, other things equal, 

 each of these attributes, giving its possessor an extra chance of life, is likely to 

 be transmitted to posterity. But there seems no reason to suppose that it will 

 be increased in subsequent generations by natural selection. That it may be 

 thus increased, the individuals not possessing more than average endowments of 

 it, must be more frequently killed off than individuals highly endowed with it; 

 and this can happen only when the attribute is one of greater importance, for 

 the time being, than most of the other attributes. If those members of the spe- 

 cies which have but ordinary shares of it, nevertheless survive by virtue of other 

 superiorities which they severally possess ; then it is not easy to see how this 

 particular attribute can be developed by natural selection in subsequent genera- 

 tions. The probability seems rather to be, that by gamogenesis, this extra en- 



