VARIATION 15 



of the extreme forms. Some further account of the 

 study of continuous variations will be given in 

 Chapter IV. 



The second kind of variation is variously known as 

 abnormal, definite, and discontinuous variation, and 

 includes what are known as sports and mutations. 

 Such variations, as is indicated by the terms applied 

 to them, involve definite differences usually of con- 

 siderable amplitude. A good example of a discon- 

 tinuous variation would be afforded by the appear- 

 ance of a child having six fingers in a family in which 

 this abnormality was not previously known to occur. 

 We shall pursue the discussion of discontinuous varia- 

 tion and of the methods of perpetuation of the types 

 which thus arise in Chapters V. and VII. 



A short account of the historical development of the 

 theory of organic evolution is given in Chapter II. It 

 is of particular interest to notice that the modern view 

 of the mutationist is foreshadowed with remarkable 

 precision in the passage from Aristotle's ' Physics ' 

 which is quoted in Chapter V. Passing to more recent 

 times, three distinct accounts of the method of origin 

 of specific differences have been proposed almost within 

 the last century, and each of these theories still finds 

 a number of supporters. 



i. The view of Lamarck, published first in 1801, and 

 in an enlarged form in 1809, was briefly as follows : 

 Noticing that the organs of men and other animals are 

 increased and strengthened by use, and particularly 

 by conscious use, Lamarck assumed that this effect 



