424 THE PREFUNCTIONAL AS CONTRASTED WITH 



level. Within vertebrate limbs, development takes place centri- 

 fugally, and, as a result of this, the later- differentiating parts will 

 increase in proportionate size during development. ^ Fourthly, 

 there are growth-processes directly concerned with the functional 

 demands made upon an organ : these also involve change of pro- 

 portion and will be dealt with later. Fifthly, the growth of one 

 structure may be modified by the specific growth-rate of neigh- 

 bouring structures. 



In illustration of this last point, it is found that the structures 

 composing the eyes in Amhlystoma punctatmn and tigrinutn have 

 diflFerent specific growth-intensities (Chap, x, p. 366). By making 

 grafts of eye-cups and of lens-forming epidermis between these two 

 species, it is found that the presence of a fast-growing eye-cup is 

 correlated with an increase in the growth-rate of a slow-growing 

 lens associated with it, and vice verso? (fig. 204). 



Mechanical modification of growth-processes is readily brought 

 about. It is only necessary to recall the artificial deformations of 

 skull, lips, waist, feet, etc., practised by various human societies. 

 In this connexion may be mentioned the fact that when axolotl 

 larvae are reared in dishes with only a little water so that their 

 backs protrude above the level of the water, the dorsal fin falls over 

 owing to its weight, and becomes completely united to the skin of 

 the back. But, internally, the structural and histological features 

 of this finless condition are quite distinct from those produced as 

 a result of normal metamorphosis,^ although externally they are 

 more or less similar (fig. 205). 



§3 



The unification of the organism by means of the nervous system 

 brings the various parts into more intimate relations with each 

 other as regards their functional activities, and brings the organism 

 as a whole into a more intimate and more delicately adjusted re- 

 lation with the environment. This is responsible for a greater 

 dehcacy of functional adjustment on the part of the various organs. 



^ See Huxley, 1932, Chap, iv. 



2 Harrison, 1929; Twitty 1930; Twitty and Schwind, 1931. This is also true 

 of limbs (Rotmann, 193 1, 1933), but does not happen with parts of the shoulder- 

 girdle. ^ Huxley, 1925. 



