70 PROBLEMS OF RELATIVE GROWTH 



again of course obtain a bimodal curve for the whole population 



(Fig- 39)- 



This can only mean that the change from narrow to broad 



type is effected during a single instar; and further that it 

 must be initiated at a relatively constant phase of the instar, 

 for otherwise there would be greater variability in the broad- 

 type abdomen than is actually found. Finally, the fact that 

 the bimodal curves are found over a considerable range of 

 body-size indicates that there is not one particular serial 

 number of moult, or one limited body-size, at which the 

 transformation (which presumably is associated with sexual 

 maturity) is initiated. If the high growth-ratio of the abdomen 

 had extended over two instars, we should have had a trimodal 

 instead of a bimodal curve ; but the more instars over which 

 it extended and the greater the variation in the body-size at 

 the initiation of heterogony, the more obscured would the 

 curve's multimodality become, owing to the variation in the 

 amount of growth at each moult, which becomes cumulative 

 with the increase in the number of moults concerned. 



A similar but less-marked bimodality occurs in this species 

 as regards chela-size in males, and is doubtless to be explained 

 in the same way ; see also Table IIIa, p. 54, for a similar 

 phenomenon in /. mauritanicus . 



The important point to notice is that the unusual and 

 apparently abnormal fact of dimorphism in one sex has here 

 been brought about by a combination of the two normal and 

 common processes of moulting and heterogony. 



It will at once be seen that this type of dimorphism is 

 quite different from that noted by G. Smith (1. c.) which we 

 have just considered in regard to the chelae of males of the 

 same genus. The ' low ' and ' high ' males in this case were 

 simply the groups of small and large body-size in what would 

 have been a case of continuous heterogony resembling that 

 of the chela of male Uca, if it had not been for the intercalation 

 of a non-breeding period in which the claw reverted to female 

 type. This dimorphism has nothing to do with moulting, 

 but is due to an interruption of a long-continued heterogony 

 by a non-breeding phase ; while that of the female abdomen 

 of the same genus is due to the restriction of heterogony to 

 a single moult-period. Both, however, are dimorphisms of 

 developmental origin, and have nothing in common with 

 genetic dimorphisms like those of ' diphasic ' mammals or 

 birds such as arctic fox, certain squirrels, herons, owls, etc., 



