296 Mr Punnett, Note on the proportion of the sexes etc. 



curve are shewn graphically the relative proportions of the two 

 sexes at different stages of growth. Until a carapace breadth of 

 15 mm. is reached, £ and % crabs exist in equal numbers. After 

 this the % s begin to preponderate, until, for a carapace breadth of 

 36 — 40 mm., they are more than twice as numerous as the </ s. 

 When the carapace breadth exceeds 50 mm. the Js outnumber 

 the other sex. This is due to the fact that the </ attains a some- 

 what greater size ; but even when allowance is made for this the 

 great preponderance of $s from 15 mm. onwards still remains. 

 So it would appear that whilst in very young crabs the sexes 

 exist in equal numbers, the mortality among the ^s in subsequent 

 stages is greater than that which occurs among the % s. 



This increase of male mortality during the later stages of 

 growth is probably due to the larger number of risks which they 

 run owing to their more active habits. An observer stationed on 

 the rocks will frequently notice crabs scurrying about near the 

 edge of the incoming tide. In almost every case such crabs will 

 turn out to be males. The % on the other hand, burdened as she 

 so frequently is with the weight of a future generation, is inclined 

 to more sedentary habits. Whilst collecting I have noticed that 

 the great majority of crabs which require to be dug out of the 

 sand are members of this sex. 



Consequently it may, I think, be fairly concluded that (1) during 

 the early stages of growth the proportion of the sexes is equal, and 

 that (2) the approach of sexual activity is accompanied by changes 

 of habit and disposition which, by exposing the males to greater 

 risks, lead to an increased mortality during later stages of growth in 

 this sex as compared with the $s. If we assume that the death 

 rate in the larval and early post-larval stages is equal for the two 

 sexes, then the former of the above two conclusions is in accord- 

 ance with the view that Mendel's law applies to sex heredity in 

 the crab. The second conclusion suggests the danger of drawing 

 any conclusion as to. the relative numbers in which the sexes are 

 produced from the proportion of the sexes at later stages of growth. 

 It further hints at a possible connection between sexual di- 

 morphism and a different sexual mortality rate. But until more 

 cases of a similar nature have been investigated it would be 

 unprofitable to dwell further on this point. 



