II] VARIATION 19 



the pale colour is entirely distinct from the cause 

 of the absence of pigment in the white varieties of 

 those species. 



The recognition of the importance of discontinuity 

 in variation, which we owe chiefly to the work of 

 Bateson in England and De Vries in Holland, is one 

 of the chief advances which the study of the subject 

 has made since the time of Darwin. 



One other distinction between different kinds of 

 variation must be mentioned here, which will be 

 discussed more fully in subsequent chapters. The 

 kinds of variation mentioned above are all inborn, or 

 inherent in the individual and to a great extent in- 

 dependent of its manner of life. But it is well known 

 that the continued use of an organ or structure, or the 

 prolonged action upon it of some external stimulus, 

 may alter its form or cause it to assume a condition 

 different from that which it would have had if these 

 influences had not acted. In general, an organ tends 

 to adapt itself either to the uses to which it is put or 

 to the action of the environment which surrounds it. 

 The muscles of a limb used for strenuous work in- 

 crease in size and strength, or a part of the skin 

 continually exposed to bright light develops a deeper 

 colour than if it is covered. The converse process is 

 also true ; an organ which is not used or exposed to 

 its normal stimuli tends to diminish, and become less 



2—2 



