The Theory of Evolution 219 



polyhedron may roll over on to another face, which would corre- 

 spond to the change of a species into a new one. Should the 

 polyhedron be so constructed that one of the new faces is much 

 more stable, or easily reached, we can understand' why in some 

 cases only a single mutation is Hkely to occur when the balance is 

 upset. Such would be the case with the turnspit type of dog, 

 the Japanese peacock, the merino sheep, the short-horned cattle. 

 If, on the other hand, one new face is as likely as the other to 

 give a stable condition, results like those in the primroses would 

 seem to be produced. We might even go farther and conclude 

 that this has been the case with the two hundred elementary 

 species of Draba vema, with the many elementary species of the 

 wild apple, and possibly with the races of domesticated pigeons. 

 The analogy need not be pushed too far, nor be taken for more 

 than it is worth. 



De Vries attempts to draw a sharp line between elementary 

 species and varieties, and while the distinction is useful, yet, on 

 the whole, it seems rather arbitrary. A variety is a type in 

 which one of the parent-species characters has disappeared — 

 not necessarily absolutely, for it may remain only latent in the 

 germ-cells, but it never develops or only under unusual condi- 

 tions.^ As an example, an albino mouse is a variety of the house 

 mouse. It is characterized by the loss of a character — the 

 pigment. Nevertheless as we have seen in the preceding chap- 

 ters the character that disappears may really be only latent in 

 the germ-cells. 



It is evident that it may often be convenient to distinguish 

 between new types in which a new character or combination of 

 characters has appeared, and new types distinguished only 

 by the loss of a character. ^The distinction may seem to be one 

 of only minor importance were it not that it has been held to 

 separate new types that obey quite different laws of heredity. 

 It is sometimes said that varieties, in de Vries's sense, follow 

 Mendel's law, while elementary species do not. De Vries has 

 indicated how he supposes this difference might come about. 



^ Hybridization often brings such latent characters once more to the front. 



