CAUSES OF VARIATION. 2Q 



external conditions. He assumed, therefore, as a 

 basis for his theory, that all organisms have an in- 

 herent tendency to vary. In accordance with this 

 tendency, every individual departs more or less from 

 the form of its parents. Such departures he believed 

 were indefinite and irregular, simply being the re- 

 sult of a tendency to change. Darwin came to this 

 view because he was seldom able to trace any definite 

 connection between the environments of an organ- 

 ism and its conditions. Another school, which is 

 called the Neo-Lamarckian school, does find a 

 definite connection between the organism and its 

 environment. The representatives of this school 

 point to instances increasing every day, where it has 

 been shown that definite changes in condition in- 

 duce definite changes in animals. They show that 

 many animals, under the same conditions, vary in 

 the same or in parallel directions. They point to 

 numerous instances where changes in locality pro- 

 duce definite changes in structure such as the case 

 above mentioned of American birds. They con- 

 clude, therefore, that we are to look at the environ- 

 ment exclusively for the cause of variation ; and 

 they say that if we find any series of variations ac- 

 cumulated generation after generation, it is because 

 the same external conditions continue to produce 

 them. Some naturalists believe in an internal law, 

 which is supposed to have charge over variations, 

 and to regulate them independently of external con- 

 ditions. In accordance with this supposed law, any 

 amount of variation may appear suddenly, without 

 any external factor for bringing it into existence. 



