Ill 



OF DIMORPHIC GROWTH 



213 



phenomenon was described, and has been often quoted, as one 

 of dimorphism or discontinuous variation. In this diagram the 

 time-element does not appear; but 'it looks as, though it lay close 

 behind. For the two size-groups into which the tails of the earwigs 

 fall look curiously hke two age-groups such as we have already 

 studied in a fish, where the ages and therefore also the magnitudes of a 

 random sample form a discontinuous series (Fig. 59). And if, instead 

 of measuring the whole length of our fish, we had confined ourselves 

 to particular parts, .such as' head, or tail or fin, we should have 

 obtained discontinuous curves of distribution for the magnitudes 



200 



150 



100 



Ocm. \5 20 25 30 



Length in cm. 

 Fig. 59. Length of body in a random sample of plaice. 



of these organs, just as for the whole body of the fish, and just as 

 for the tails of Bateson's earwigs. The differences, in short, with 

 which Bateson was dealing were a question of magnitude, and it 

 was only natural to refer these diverse magnitudes to diversities of 

 growth; that is to say, it seemed natural to suppose that in this 

 case of "dimorphism," the tails of the one group of earwigs (which 

 Bateson called the "high males") had either grown faster, or had 

 been growing for a longer period of time, than those of the "low 

 males." If the whole random sample of earwigs were of one and 

 the same age, the dimorphism would appear to be due to two 

 alternative values for the mean growth-rate, individual earwigs 

 varying around one mean or the other. If, on the other hand, the 



