212 Life and Death, Heredity and Evolution 



possibility of getting therefrom by selection various grades 

 of a given external characteristic. In this, so far as I can 

 see, there is complete agreement. 



It appears then that under the recent careful studies 

 made, it can no longer be maintained that hereditary changes, 

 even in higher organisms, must be large leaps or saltations. 

 They may be of this character, but they may equally well be 

 graded changes so slight as to be hardly detectible when 

 taken singly. 



This appears to be recognized by those who have proposed 

 and defended the mutation theory. De Vries (1916) in a 

 recent summary of the theory emphasizes throughout exten- 

 sive mutations, and speaks repeatedly of their origin as 

 "sudden and without transitional conditions," but admits 

 also that t( not only very small, but also much greater" dif- 

 ferences between species arise all of a sudden ("mit einem 

 Sprunge") ; and that most mutations affect only a single 

 character. He sets forth further that in a single stock one 

 such mutation after another may arise, at intervals, until in 

 the course of time the stock has become very diverse from 

 the original one; and has become differentiated into a num- 

 ber of different types on which selection may act. Now, so 

 far as the mutations are "very small," the condition after 

 but one or a few mutations had appeared would be prac- 

 tically indistinguishable from a "transitional condition" to 

 the state after many mutations had occurred. Morgan 

 (1917) recently insists that it must be recognized and has 

 always been urged by de Vries that "mutations may be very 

 small so far as the character change is concerned." 



The true and important points insisted on by the muta- 

 tion theory appear to be these: 



(1) There are many differences ("variations") between 

 individuals that are not heritable. Hence by selection of 

 such diversities no evolutionary change is produced. 



