78 EVOLUTION OF LIVING OEGANISMS. 



Darwin used to say that a single instance of such useless 

 evolution would be fatal to his whole theory : it need 

 hardly be added that none has ever been found. 



Natural selection can never be prophetic ; organs can- 

 not be developed before they are needed. Certainly 

 organisms may on rare occasions by chance, so to speak, 

 find themselves ready adapted to meet new conditions ; 

 so protoplasm may possess properties which have never 

 yet been made use of in the struggle for existence as, for 

 instance, the power of responding to galvanic stimuli. 

 But every step in the process of evolution by selection 

 must of necessity be useful, must lead to survival. 



Apparent exceptions to this rule may be due to corre- 

 lation. Factors of inheritance are able to affect not one 

 but several characters, or even the whole organism, so 

 that alteration of a character by selection may lead to 

 correlated alterations in other parts. Thus changes may 

 be brought about not for their own sake, and characters 

 useless or even harmful may be developed, so long as the 

 advantage gained is not counterbalanced. Correlation 

 plays an important part in evolution. It has been 

 elaborately studied mathematically by Pearson and 

 others, and has been proved to occur extensively, and 

 often in most unexpected directions. 



At the same time, the tendency shown by the detrac- 

 tors of Darwinism to assert that this or that character is 

 -seless, because they cannot find a use for it, is strongly 

 ; i be deprecated. Every day naturalists are discovering 

 the functions of the most insignificant-looking organs. 

 Little importance can be attached to the statement often 

 made that the characters which distinguish nearly allied 

 species are of no value to them ; in fact no character 

 should be accepted as useless until it has been definitely 

 proved that it exerts no influence on the death-rate. 

 Some few years ago it might have been held indeed it 

 was held that such organs in man as the thyroid gland, 

 the pituitary gland, the suprarenal glands, and others, 



