MUTATIONS 57 



is to say, before they threaten the constancy of the species. 

 The importance of the role played by germinal selection in the 

 elimination of plus variations is proportionate to the fixity of 

 the germ-plasm, as in the case of constant and ancient species. 

 In the case of new, and consequently less constant, species this 

 role of germinal selection is less important, for, the germ-plasm 

 being less fixed, the determinants are more likely to gain a plus 

 variation in a majority of the ids of a large number of indi- 

 viduals. 



Every plus variation of the determinants of a given organ 

 entails a minus variation of other determinants, because of the 

 limitation of the quantity of intragerminal nutrition ; and every 

 group of determinants which finds itself in a course of ascendant 

 variation tends to attract an ever-increasing supply of nutri- 

 ment, so that the regressive variation of the rival group is thereby 

 more accentuated. There is a maze of intricate and complicated 

 relations between the component parts of the germ-plasm, and 

 the perturbations which take place continually in the balance 

 of these component parts constitute the origin of aU hereditary 

 variations. Such variations may be of two kinds : sudden varia- 

 tions, which affect at the same time a large number of indi- 

 viduals ; or gradual variations, slow transformations taking place 

 within the germ-plasm of single indiAriduals, or of a few, and 

 related to gradual adaptation to environing conditions. De 

 Vries has particularly insisted on the sudden and more wide- 

 spread variations, which he has aptly termed " mutations." Such 

 mutations are especially frequent among plants, as the experi- 

 ments of De Vries have shown. But they are also operative in 

 the animal world. They have probably contributed in par- 

 ticular to the evolution of secondary sexual characters. In the 

 case of plants, mutations are remarkable for the pronounced 

 character of the changes they effect ; and as vividness and pro- 

 nounced character are essential features of secondary sexual 

 distinctions which are observable especially in birds and certain 



