Ill 



A CASE OF DIMORPHISM 



105 



tail or fin, we should have obtained discontinuous curves of 

 distribution, precisely analogous to those for the entire animal. 

 Now we know that the differences with which Bateson was dealing 

 were entirely a question of magnitude, and we cannot help seeing 

 that the discontinuous distributions of magnitude represented by 

 his earwigs' tails are just such as are illustrated by the magnitudes 

 of the older and younger fish ; we may indeed go so far as to say 

 that the curves are precisely comparable, for in both cases we see 

 a characteristic feature of detail, namely that the "spread" of the 

 curve is greater in the second wave than in the first, that is to 



20 25 30 



Length of fish, in cm. 



Fig. 24. Variability of length of body in a sample of Plaice. 



say (in the case of the fish) in the older as well as larger series. 

 Over the reason for this phenomenon, which is simple and all but 

 obvious, we need not pause. 



It is evident, then, that in this case of "dimorphism," the tails 

 of the one group of earwigs (which Bateson calls the "high males") 

 have either grown faster, or have been growing for a longer period 

 of time, than those of the "low males." If we could be certain 

 that the whole random sample of earwigs were of one and the 

 same age, then we should have to refer the phenomenon of di- 

 morphism to a physiological phenomenon, simple in kind (however 

 remarkable and unexpected) ; viz. that there were two alternative 



