186 



The former iiuiy all be aceouiited tor by vefereiiee to the action of uat- 

 ural selection. Given a parasite with an inherent tendency to varia- 

 tion in all directions, some characters will appear that give their pos- 

 sessors some slight advantage over their fellows. Xatural selection 

 seizes upon them at once, intensifies them by the elimination of the 

 less well-endowed individuals, until what was perhaps at first only a 

 slightly longer or more strongly curved claw becomes eventually the 

 elaborate apparatus for clinging to hairs which, some mites possess. 

 The roughened skin and forcipate pal]>i can be accounted for in mucli 

 the same way. 



It is when we come to characters of the second group, those we are 

 accustomed to consider the result of disuse, that the difficulty begins. 

 If,as Weismann and his disciples claim, the direct results of disuse which 

 appear in the individual are not transmitted, how is it that mites come 

 to have no eyes and in some cases i)ossess greatly reduced limb.^? As 

 has been admitted by Weismann, it is not clearly apparent how the pres- 

 ence of eyes is a disadvantage to a parasitic or a cave animal; and if 

 it is not, why should they disappear? 



If we accept Weismann's views on the transmission of somatogenic 

 characters, we must hold that in some roundabout way these efl'ects of 

 disuse, become blastogenic. But if disuse may affect the germ and its 

 effects be transmitted, why may not use also be felt and its effects become 

 hereditary ? Weismann would account for the reduction of parts by su])- 

 posing that when an organ ceases to come under the influeuce of natural 

 selection those individuals v;ith the organ least developed have the 

 same chances, other things equal, as those with it most developed, and 

 that consequently by intercrossing the organ is reduced from its highest 

 state of j)erfection. 



It seems plain that when selection ceased to act the organ Avould be 

 reduced (and elevated) to the average of development for the species, 

 but without selection it could not be reduced below this average — would 

 not consequently disappear. Its variation would be in all directions, 

 and the chances for an increase in complexity and fun(>tional activity 

 the same as for a decrease. 



We know, however, that organs which from some cause have become 

 useless do tend to disappear, and the reason for their disappearance, 

 if we can find it, will throw light on some of the peculiarities of the 

 Sarcoptidse. 



We can see a disadvantage in oneway in the possession of a complex 

 but useless structure. It costs material and energy for its development 

 Avhich could be profitably expended in other directions. An associa- 

 tion mjide up of organs, everyone of which is useful, has an advantage, 

 though it may be slight, over an association a part of whose energy 

 goes to the support of useless members. The struggle for existence is 

 keen. Individuals whose energy is all utilized grow faster, are more 

 prolific, more energetic, better able to take care of themselves, and in 



