138 THEORIES OF HEREDITY 



more probably due to the cessation of Natural Selection, 

 which can no longer maintain the efficiency of a useless 

 part. All functional parts of an organism are kept up 

 to a high standard by the operation of Natural Selection ; 

 withdraw selection and sooner or later degeneration will 

 begin. It is very interesting to find that both Galton 

 and Weismann independently arrived at the conclusion 

 that the cessation of Natural Selection offered a better 

 explanation of the gradual dwindling of useless parts, 

 than that afforded by the supposed transmission of the 

 admitted dwindling which follows from disuse during 

 an individual life. 



Finally, the phenomena of instinct seem capable of 

 explanation by the operation of Natural Selection upon 

 blastogenic variations of the nervous system, rather than 

 by the supposed transmission of acquired habit. In 

 many cases we are compelled to adopt the former theory, 

 and it is open to us in all. 



The time at our disposal has made it impossible to 

 attempt any real discussion of the transmission of acquired 

 characters. I have only indicated the lines along which 

 it is likely that discussion will be directed. 



Note. — Mr. Francis Galton kindly writes to me (Feb. 12, 1907) on 

 the subject of the first paragraph on p. 134 : — 'There is plenty of evidence 

 that twin-bearing runs in families, but I know of no inquiry as to whether 

 the tendency to produce identical twins does so. It would be a hard task 

 to collect and to sift adequate evidence on this point.' 



