ON THE CAUSES OF VARIATION. 811 



modification affecting the mass of individuals of a species through 

 the environment ; because the environment affects function, and 

 function in its turn affects form and structure. The life of every 

 individual furnishes an excellent illustration of new action and 

 new uses for organs not previously used, in the striking and sud- 

 den employment of post-natal organs, both of respiration and 

 nourishment, which pre-natally had no corresponding action. Ko- 

 manes has argued that cessation of selection may reduce an organ 

 where use or disuse can have no play, as in the loss of wings in 

 neuter ants ; and that by the law of compensation an organ may 

 even be increased, as in the heads of such neuters. He enforces 

 the idea by exampling the blind crabs of our Kentucky caves, 

 where the complex eyes rapidly disappear under cessation of se- 

 lection, but where the persistence of the foot-stalks indicates that 

 economy of nutrition could have had little play ! It is difficult, 

 however, to draw the line between this cause and Lankester's 

 reversal of natural selection ; and still more difficult to say wherein 

 either differs from mere disuse. 



Degeneration, which has been urged as the true explanation of 

 many of the existing forms of life, is, it seems to me, but a con- 

 sequence of disuse, and would, therefore, fall into the present cate- 

 gory, among causes of variation. 



Emotion as ajfecting the Individual. — I have here considered 

 the factor of use and disuse as a direct cause of variation, from 

 the psychical rather than the physical standpoint — i. e., individual 

 or conscious effort as furnishing food for natural selection, among 

 more highly endowed animals, rather than as effort by species as 

 a whole necessitated by physical conditions, and inducing modifi- 

 cation in masses irrespective of selection. This leads us to the 

 consideration of mind as a factor in evolution, and we shall soon 

 see its importance as a fundamental cause of differentiation, 

 among higher organisms at least. I am not sure, even, that its 

 influence can be excluded from among lower animals, however 

 much we may have to exclude its action in so far as plants are 

 concerned ; for any new functional effort inducing new use may 

 be looked upon as conscious and intelligent as compared with use 

 fixed by habit and lapsed into automatic action or instinct. The 

 former typifies variability and progress ; the latter constancy and 

 stability. 



Mind is a comprehensive cause of variation, and may be con- 

 sidered under several categories. We have, for instance, (1) the 

 action of the mind of the individual in willing, or in selecting 

 between differing alternatives that present themselves, as in the 

 choice of means to ends ; (2) the direct influence of the emotions 

 on the individual ; and (3) the influence of the emotions of the 

 mother on her unborn offspring. 



