CHAPTER VII 



BEARINGS OF THE STUDY OF RELATIVE 

 GROWTH ON OTHER BRANCHES OF 



BIOLOGY 



IT will be clear from preceding chapters that the study of 

 relative growth has important bearings upon many other 

 branches of biology. In the present chapter I will attempt 

 to summarize a few of these as succinctly as possible. We 

 will begin with its bearing upon systematics. 



§ i. Heterogony and Taxonomy : Sub-species and 



Taxonomic Forms 



Systematists are agreed that mere size-differences may have 

 no taxonomic significance, since they are often the direct 

 effects of environmental conditions. But they usually attach 

 much greater importance to differences in the percentage size 

 of parts. Our studies of heterogony, however, make it obvious 

 that such differences may have precisely as little taxonomic 

 significance as those in absolute size, for wherever an organ 

 is heterogonic, differences in absolute total size of body will 

 bring about differences in relative size of the part. 



When such problems crop up, they require analysis along 

 the following lines : 



(i) Is there a difference in total absolute size between 

 the varieties considered ? 



(2) If so, is the difference due (a) wholly to environmental 

 differences, (b) wholly to genetic differences, or (c) to a com- 

 bination of the two ? 



(3) Is there a difference in the relative size of any parts or 

 organs ? 



(4) If so, is there also a difference in total absolute size ? 



(a) If not, heterogony is not at work, and the difference 



is genetic and is of taxonomic value. 



(b) If yes, then do the relations of relative size of part to 



absolute size of whole obey the simple heterogony 



law ? 



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