256 



ESSENTIALS OF BIOLOGY 



sent the frequency distribution of some variable character in a 

 population of the same race : Let this character be such that 

 if it varies above the average it will confer advantage on the 

 variant. 



The mean value of the variable character is exhibited by the 

 group a. The groups to the right of a display the variable 

 character in higher degree than do those at a. Conversely the 

 groups to the left of a display the character to a lesser degree. 

 All the groups to the right of a, say those at h, have therefore 

 some advantage over the a's in the conditions of the struggle for 

 existence — they are the " fitter." And therefore more of the 6's 



B 



cu 



Meaiv 

 I 



B' 



Meoji' 



X 



MeaiL Mecuv 



Fig. 35. — Diagram showing Regression. 



will survive and reproduce than their numerical proportions in 

 the distribution would suggest. Conversely less of the variants 

 to the left of a, say the c's will survive and reproduce. 



But the ^'s have displayed a variation that is advantageous and 

 which we assume to be hereditary. Therefore the progeny of 

 all the h's that have reproduced will also display this variation. 

 Let us, for simplicity, take the extreme case : the new frequency- 

 distribution of the value of the variable character is represented 

 by the graph B and this shows that the advantageous character, 

 which was exceptional in its occurrence in ^, is now at the mean 

 in B. That is, the exceptional variants in A are now the ordinary 

 individuals of B. Clearly transformism has begun and will 

 continue, for the same process of advantageous variability, and 

 the transmission of this to another generation, will begin anew 

 from the exceptional variants of B. Thus a new category of 



