186 
The former may all be accounted for by reference to the action of nat- 
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. Natural 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 foreipate palpi can be accounted for in much 
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 eases possess greatly reduced limbs? 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 effects 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 thereduction of parts by sup- 
posing that when an organ ceases to come under the influence of natural 
selection those individuals with 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 perfection. 
It seems plain that when selection ceased to act the organ would 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 functional 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 
Sarcoptide. 
We can see a disadvantage in one way in the possession of a complex 
but useless structure. It costs material and energy for its development 
which could be profitably expended in other directions. An associa- 
tion made up of organs, every one 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 
