40 PROBLEMS OF RELATIVE GROWTH 



of proportions with absolute size, and to proceed as if certain 

 arithmetic (percentage) proportions were immutable ' charac- 

 ters ' of the species, may lead to serious error. 



But we must remember that the limitation of growth and 

 the consequent establishment of a small range of stable adult 

 size is a late and specialized feature in evolution. The majority 

 of animals show unlimited growth : they continue growing, 

 though usually at a constantly diminishing rate, until they 

 die, or in asexually-reproducing forms, until they divide. A 

 lobster or a plaice may increase its linear dimensions several 

 fold after the attainment of sexual maturity. In such types, 

 there is no fixed or adult form ; the change of proportions 

 continues unabated throughout life, and may be as obvious 

 during post-maturity as during pre-maturity. An excellent 

 example of this is provided by the detailed studies of Mrs. 

 Sexton (1924) on the successive instars of Gammarus chevreuxi, 

 supplemented by the work of Kunkel and Robertson (1928) 

 on the same species. 1 



Even among mammals a change of proportions may con- 

 tinue throughout life. In the voles (Microtinae) Hinton (1926, 

 Chap. II, 8-14, Pis. Ill, IV, IX) finds that slow growth occurs 

 long after the adult state has been arrived at, the epiphyses 

 of the long bones never uniting. This continuous growth is 

 accompanied by continuous change of proportions. With 

 increasing size of the skull, for instance, the rostrum becomes 

 relatively narrower and slightly longer, the interorbital region 

 narrower and the molars relatively smaller. Unfortunately 

 the measurements given do not permit of any accurate state- 

 ment as to the changes involved, or as to the distribution of 

 growth-potential in different regions. Here is an interesting 

 field for the student of relative growth. It would be par- 

 ticularly interesting to discover whether the relative growth- 

 rates of tail and parts of skull, limbs, etc., remained the same 

 after the attainment of sexual maturity as they did before. 

 It would be easier to investigate this on the limb-segments 

 than on the skull, which undergoes complex distortions and 

 curvatures. It would also, of course, be necessary to keep 



1 Sexton states that sexual maturity occurs at the seventh instar, 

 that proportions continue to change for two further instars in the 

 male, one in the female, but after this no further proportion-changes 

 occur (though the males at least may increase about 40 per cent, in 

 length). That this statement is not accurate is shown by Kunkel and 

 Robertson, whose graphs demonstrate a change in the proportions of 

 several organs up to the largest sizes found. 



