216 PROBLEMS OF RELATIVE GROWTH 



between wild species would represent the best possible physio- 

 logical adjustment between brain-size and body-size. We 

 should also expect that the genetic variations in body-size 

 within a wild species would also presumably be largely inde- 

 pendent of those affecting absolute brain-size ; but so far as I 

 know, data on this point are lacking. (See also p. 224 ; heart.) 

 Whether any such discrepancy between intra- and inter- 

 specific formulae would be found in regard, e.g., to the horns 

 or mandibles of male beetles remains to be seen. If it did 

 occur, it might well be expected to be of opposite nature, the 

 larger species having relatively smaller horns than would be 

 expected from the extrapolation of the intra-specific curve of 

 smaller species. But this, as I have already said, also remains 

 to be determined. 



§ 3. Heterogony and Evolution 



A further evolutionary implication of these facts is this : 

 that the existence of an organ with high growth-ratio tends 

 to limit the extreme size attainable by the type in question 

 during evolution, for with very large absolute size, the 

 relative size of the organ will tend to become so huge that 

 it becomes unwieldy or even deleterious. Doubtless the organ 

 could be held in check by a modification of its growth-ratio 

 such as we have been envisaging. But for increase of absolute 

 size, two concomitant processes of mutation would then have 

 to occur — mutations favouring decreased growth-ratio of the 

 organ, as well as those favouring increased absolute body-size, 

 so that the evolutionary problem is thereby complicated. 



It can be calculated that a male Uca with body weighing 

 1 kg. would, if its claw's growth-ratio had remained unaltered, 

 boast a large chela weighing some 10 kg. — which, as Euclid 

 says, is absurd : and it is perhaps no coincidence that the 

 largest fiddler-crabs attain sizes far below those of many 

 other Brachyura, and even far lower than those of other land 

 or semi-land crabs (Ocypoda, Birgus, etc.) ; see p. 32. The 

 excessive antlers of the Irish ' elk ', Megaceros, may very 

 likely be accounted for on similar grounds. An organ which 

 is verging on the deleterious may rapidly become actually 

 deleterious if conditions change. 



A further important modification of the rule has been 

 pointed out by Champy (1924, p. 156 seq.), viz. that it applies 

 only to the size of the organ. The details of form may be 

 radically modified, and yet the general size conform to the rule. 



