266 Evolution and Adaptation 



Thus the mode for beech trees in general is sixteen ; but, as 

 shown in the other table, this mode does not correspond with 

 either of the two individual modes here ascertained. The 

 illustration shows that the racial mode may differ from the 

 individual mode. There are also cases known in which 

 the mode of a group of individuals living in one locality is 

 different from that of another group living in another locality. 

 This difference may be a constant one from year to year, 

 although so slight, that unless actual measurements are made, 

 the difference cannot be detected, because of the overlapping 

 of the individuals from different localities. If evolution took 

 place by slow changes of this sort, it might be possible to de- 

 tect its action, even when very slow, by means of measure- 

 ments made on a large number of individuals. At least this 

 has been suggested by those who believe new species may 

 result from changes of this sort. 



There is some evidence showing that by selecting particular 

 individuals of a series, and breeding from them, the mode 

 may be changed in the direction of selection. Thus it has 

 been stated by Davenport that the descendants of twelve- 

 and thirteen-rayed daisies give a polygon with a skewness of 

 + 1.92; while the descendants of twenty -one-rayed plants 

 give a polygon with a skewness of —.13. 



Pearson has described very concisely the possibilities in- 

 volved in the selective action of the environment. He states 

 that if we examine the frequency distribution of a set of 

 organisms that have just become mature, and later make a 

 similar examination on the same number of individuals (but 

 not the same individuals) during the period of reproduction, 

 we shall probably find that a change has taken place which 

 may have been due to selection of some sort. The same 

 thing might be found in the next generation, and, if it did, 

 this would indicate that " selection does- not necessarily mean 

 a permanent or a progressive change." The selection in this 

 imaginary case would be purely periodic and suffice only to 



