74 PROBLEMS OF RELATIVE GROWTH 



The hypothesis of an extra moult may however be used to 

 account for the facts, along the following lines. The processes 

 responsible for the heterogony of the male forceps begin 

 operating, we will suppose, late in larval life, and usually at 

 a fairly definite phase of an instar. They cannot, however, 

 be expressed in the form and size of male-type forceps until 

 the imaginal instar is reached. This may be reached either 

 at the next moult after the initiation of the heterogonic process, 

 or only at the second moult. The result is that the heterogonic 

 processes have either been operating for less than one instar, 

 or for a period more than double as long, with resultant 

 bimodality. Variation in the intensity of the heterogony, 

 and in the time after moulting at which it is initiated, will 

 bring about the overlap of high and low ; there is, however, 

 no qualitative difference to be expected between the two 

 forceps-forms (Fig. 41). 



The interesting fact discovered by Djakonov (1. c.) should 

 be noted, namely that unfavourable nutritive conditions 

 cause a decrease in mean and modal body-size, both for the 

 population as a whole, and for the ' low ' and ' high ' groups 

 considered separately. As regards forceps-length, however, 

 the main effect is to cause a shift in the distribution of the 

 forceps, there being fewer high-type and more low-type, but 

 without alteration of either modal value (Fig. 42). Furthermore, 

 though in unfavourable conditions there are fewer large indi- 

 viduals in both high and low series, yet for classes of the same 

 mean body-size, whether we consider the population as a 

 whole or the high and low groups separately, the mean forceps- 

 size is actually greater for the animals which have grown up 

 in the unfavourable conditions. 



This apparently paradoxical fact may be explained in terms 

 of our previous discussion. The unfavourable conditions 

 reduce the total body-size. But the initial growth-ratio of 

 the heterogonic forceps, starting at a late moult, will always 

 be the same ; it is only its final value which will tend to a 

 limit imposed by the growth-partition coefficient. For most 

 body-sizes therefore we should expect that the absolute size 

 of the forceps would depend only on the time for which it had 

 grown, not on the body-size at which it began to grow, and 

 therefore with stunting of total bulk, a given body-size will 

 show an absolutely as well as a relatively greater forceps- 

 size. It is, further, quite possible that the equilibrium-size 

 and final theoretical growth-ratio is in practice never attained, 



