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Geoflfrey Smith 



whole chela that is arrested at the middle stage. Now among the 

 middle males, there also occiir a veiy small number of crabs 

 measuriug between 16 and 19 mm. in carapace length, which exhibit 

 a swollen chela intermediate between the low and high males. The 

 very low frequency with which such crabs occur is exhibited in the 

 Frequency Curve Diagram 2. 



Carapace length in millimeters. 



o 



Textfigure 5 (Diagram 2). 



This curve shows the frequency with which males of all sizes with 

 swollen chelae occur duriug the breeding season, the middle males 

 with flat chelae being omitted. It is seen that there are two modal 

 points of greatest frequency, oue at 13 mm. carapace length, the 

 other at 22, and these correspond to the low and high males re- 

 spectively. Now if we dissect a large number of crabs with swollen 

 chelae during the breeding season we find that in all cases the 

 testes are very large and spermatogenesis is in full activity, but 

 the testes of males with fiat chelae are obviously reduced and sec- 

 tions show that spermatogenesis is in abeyance. The curve in 

 Diagram 2 represents, therefore, a sample of the breeding males of 

 the Population under consideratiou, which are thus seen to be essen- 

 tially differentiated into high and low forras. What then is the 

 explanation of the middle males with flat chelae, which take no 



