298 THE THEORIES OF EVOLUTION 



directions. The study of the pattern and colouring 

 of insects' wings furnishes a good many illustrations 

 of this tendency. 



We also observe that variations are not endless in 

 number nor indeterminate, but only occur in a limited 

 number of directions. "Through the very fact," 

 Plate writes, "that an animal belongs to a group, the 

 possibilities of variation are distinctly delimited and in 

 many special cases these possibilities may indeed be 

 very narrow." 1 Many cases are offered in evidence. 

 It is impossible, according to the exponents of this 

 theory, to produce blue lily of the valley, grass with 

 divided leaves, dogs with leopards' eye spots, blue or 

 green chickens, or blue or red canaries. 



Certain biologists have been led to the conclusion 

 that evolution is going on in several definite direc- 

 tions. This is what is called the orthogenetic view. 

 Theories of orthogenesis differ from one another ac- 

 cording to the factor to which scientists attribute the 

 direction followed by orthogenetic variations. 



The most representative theory of orthogenesis was 

 formulated by Th. Eimer. Radically opposed to the 

 Weismannian tendencies, he attributes to natural se- 

 lection a very unimportant role. According to him, 

 it cannot exercise itself until characters have reached 

 an advanced degree of development. Natural selec- 



i Plate. Uber die Bedeutung des Darwinschen Selectionsprincips. 

 Quoted by Kellogg: Darwinism To-day, p. 281. 



