176 7RANSMISSI0N OF ACQUIRED CHARACTERS 



during all the years in which he enjoys his pension at home. 

 He has changed his skin, but he cannot by any means 

 change it back again. Through prolonged disuse from early 

 years a muscle may pass into a state of atrophy, and may 

 so remain throughout life. Pressure on the little toe may so 

 deform it, that even in the " easiest " shoes it can never right 

 itself. A tree may be blown out of shape by the wind, and the 

 crooked bough may never be straightened. Over-exertion may 

 strain the heart permanently. A sudden shock may be followed 

 by a whitening of the hair from which there is no natural 

 recovery. 



5. Modifications and Variations. — When we analyse the 

 observed differences between fellow members of a species, we find 

 that some of them can be definitely associated with peculiarities 

 of function and environment. They can be more or less ac- 

 counted for physiologically in terms of some change in surrounding 

 influences or of some change in function thereby induced. They 

 may not be* hinted at in the young forms, but they begin to 

 appear when the peculiar conditions begin to operate, and they 

 are usually exhibited in some degree by all organisms of the same 

 kind which are subjected to the same change of conditions. 

 Furthermore, they can be experimentally brought about. These 

 are " modifications." 



By those who measure observed differences they are usually 

 slumped along with true variations, but this appears to us 

 to lead to confusion. True variations are those peculiar ities 

 which remain when all the modifications are subtracted from the 

 total of observed differences. 



It goes without saying that the distinction cannot always 

 be drawn in practice. Often, however, it is quite apparent, 

 and in any case the theoretical distinction is clear. Variations, 

 in the strict sense, cannot be causally related to peculiarities in 

 habit or surroundings ; they are often hinted at in the earliest 

 stages- — even before birth ; and they are very unequal even 



