212 



PROBLEMS OF RELATIVE GROWTH 



gressive variation of the forms as regards size has therefore 

 no ' real ' or taxonomic meaning, but is an automatic result 

 of splitting up a continuous series on the basis of purely arbi- 

 trary characteristics. (7) Thus, if you were to plot the extreme 

 variants, you would find the means for the series taken as a 

 whole keeping in the middle between the extremes, while the 

 curves for the class-means of the ' forms ' taken separately 



would each begin 

 close to the ex- 

 treme plus limit 

 of variation, and 

 end near the ex- 

 treme minus limit. 

 I have treated 

 this case also in 

 some detail, since 

 it provides an 

 excellent example 

 of the dangers 

 inherent in mere 

 labelling : although 

 the several forms 

 are actually quite 

 continuous one 

 with the other as 

 regards their diag- 

 nostic characters, 

 the mere fact of 

 having separated 

 them, though on 

 arbitrary grounds, 

 made it possible 

 to group the facts so as to make the arbitrary separation 

 appear based upon some biological reality ; and it is only 

 further analysis based upon a study of the laws of relative 

 growth and its variation which enables us to detect the fallacy. 



§ 2. Heterogony in Groups higher than the Species 



There is no need to imagine that the heterogony mechanisms 

 and other laws of differential growth are confined within the 

 boundaries of single species. On the contrary, there is every 

 reason to suppose that, like the activities of ductless glands, 

 these growth-mechanisms may operate in very similar fashion 



Fig. 93. — To illustrate change of form of man- 

 dible with increase of its absolute size in male 

 stag-beetles. 



Below, a, b, c, side view of the head in the minor, media and major 

 forms of Psalidoremus inclinatus. Above, a — 1, mandibles in the 

 genus Odontolabis. 



