238 TRANSMISSION OF ACQUIRED CHARACTERS 



exercise even in the course of a long time, and the theory requires 

 such data before it can be more than a plausible interpretation, with 

 certain a priori difficulties against it. 



Another interpretation may be suggested. If an animal suddenly 

 takes to leaping, many individual adjustments to the new exercise 

 may arise ; if the animals of successive generations leap yet more 

 freely, they may individually acquire more thorough adjustments. 

 Meanwhile there may arise constitutional variations making towards 

 adaptation to the new habit, and under the screen of the individual 

 modifications these may increase from minute beginnings till they 

 acquire selection-value (Mark Baldwin, Lloyd Morgan, and Osborn). 

 Nor should it be forgotten that variations in different parts of the 

 body are often correlated. The subsidiary theory of germinal selec- 

 tion is also helpful. Finally, it is possible that in some of these cases 

 the result was not due to the gradual accumulation of minute varia- 

 tions, but was originated by one of those sudden discontinuous 

 changes which are now called mutations. 



(2) Secondly, Spencer dwelt upon the notably diverse powers of 

 tactile discrimination possessed by the human skin, and sought to 

 show that while these could not be interpreted on the hypothesis 

 of natural selection or on the correlated hypothesis of panmixia, they 

 could be interpreted readily if the effects of use were inherited. But 

 the difficulty again is to get secure data. It is uncertain how much 

 of the inequality in tactile sensitiveness is due to individual exercise 

 and experience, though it is certain that tactility in little-used parts 

 can be greatly increased by use. Nor is it certain how much of the 

 apparent unlilieness in tactHity is due to unequal distribution of 

 peripheral nerve-endings and how much to specialised application 

 of the power of central perception. As Prof. Lloyd Morgan says : 

 " We do not yet know the limits within which education and prac- 

 tice may refine the application of central powers of discrimination 

 within Uttle-used areas. The facts which Mr. Spencer adduces may 

 be in a large degree due to individual experience, discrimination 

 being continually exercised in the tongue and finger-tips, but seldom 

 on the back or breast. We need a broader basis of assured fact." 

 Nor, it may be added, is the action of selection to be excluded. 



(3) Spencer's third set of negative evidences was based on rudi- 

 mentary organs which, like the hind limbs of the whale, have nearly 

 disappeared. Dwindling by natural selection is here out of the ques- 

 tion ; and dwindling by panmixia — i.e. the diminution of a structure 



