330 THE PLAY OF ANIMALS. 



adaptive modification will hand on to the next genera- 

 tion any " coincident variations '^ (i. e., congenital varia- 

 tions in the same direction as adaptive modifications) 

 which they may chance to have, and also allow fur- 

 ther variations in the same direction. In any given 

 series of generations, the individuals of which survive 

 through their susceptibility to modification, there will be 

 a gradual and cumulative development of coincident 

 variations under the action of natural selection. The 

 adaptive modification acts, in short, as a screen to per- 

 petuate and develop congenital variations and correlated 

 groups of these. Time is thus given to the species to 

 develop by coincident variation characters indistinguish- 

 able from those which were due to acquired modification, 

 and the evolution of the race will proceed in the lines 

 marked out by private and individual adaptations. It 

 will appear as if the modifications were directly inher- 

 ited, whereas in reality they have acted as the fostering 

 nurses of congenital variations. 



It follows also that the likelihood of the occurrence 

 of coincident variations will be greatly increased with 

 each generation, under this " screening " influence of 

 modification; for the mean of the congenital variations 

 will be shifted in the direction of the adaptive modifica- 

 tion, seeing that under the operation of natural selec- 

 tion upon each preceding generation variations which 

 are not coincident tend to be eliminated.* 



Furthermore, it has recently been shown that, inde- 

 pendently of physicial heredity, there is among the ani- 

 mals a process by which there is secured a continuity 

 of social environment, so that those organisms which are 



* This aspect of the subject has been especially eraphasizerl in 

 my own exposition, American Naturalist, June, 1896, pp. 147 ff. 



