104 THE PRINCIPLES OF HEREDITY 



because many characters which were originally absent have 

 been interpolated in the course of the life-history. 



XIX. The environment acting on, or through, the parent 

 has little or no effect on offspring subsequently born. But 

 some of the evidence on this point has yet to be considered. 

 Acting in this way it has certainly no influence on racial 

 change. Racial change, as shown by the close adaptation of 

 every species to its environment, is due to evolution pro- 

 gressive and regressive. Progressive evolution is due directly 

 to Natural Selection, which acts only when the environment 

 is injurious. Regressive evolution is due to reversion, which 

 affects a race or character only when the environment is 

 beneficial that is when Natural Selection, as regards the 

 race or character, is more or less in abeyance. In both cases 

 the racial changes are of an adaptive nature exactly opposite 

 to those which might be expected if the environment directly 

 influenced the germ-plasm. Probably Natural Selection 

 evolved this indifference of the germ-plasm to the direct 

 influence of the environment very early in the history of life 

 by selecting for survival the offspring of those individuals 

 whose germ-plasm was most indifferent. Only thus could a 

 race be rendered capable of evolving under deteriorating 

 conditions. But progressive evolution is never complete or 

 perfect, since, the moment it approaches perfection, selection 

 ceases and reversion steps in. It is probable, therefore, that 

 some offspring are still affected to some extent by powerful 

 influences acting from the environment. But, since a change 

 caused by the direct action of the environment can rarely be 

 beneficial (i. e. adaptive), such individuals, or their descendants, 

 must as a rule be eliminated sooner or later. As a con- 

 sequence the course of evolution is not affected by the direct 

 action of the environment. 



XX. Evolution is due solely to Natural Selection. But 

 besides the simple and direct elimination of the unfittest 

 which Darwin described, species are adapted to their environ- 

 ments by other means more subtle and exact. Natural 

 Selection has evolved in all plants and animals a tendency to 

 vary which decreases or increases automatically accordingly 

 as the adaptation to the environment is more or less exact. 

 The materials for the work of Natural Selection are thus 

 provided in greater or lesser abundance according to the 

 needs, for the time being, of the species. Natural Selection 

 has also evolved bi-parental reproduction which automatically 

 eliminates useless variations and structures. 



XXI. For some years past Darwin's theory of Natural 



