Page:Proceedings of the Royal Society of London Vol 60.djvu/255

234 nus and Carcinus as between two not very sharply marked off races of a single species.

Of course a considerable number of such comparisons would be necessary before any safe conclusions could be drawn, and the meaning of the differences observed could only be discovered by such a comparative treatment of a large series of genera. It is probable that the larger deviations do indicate real differences in the correlation constant, possibly such are associated with changes in habit or environment. For example, it is conceivable that a crab which swims might require to be more symmetrical than one that only crawls between the tide-marks. Portunus does swim to a certain extent, and one can see from the table that the correlation of the two sides of the body is greater in this genus than in the essentially shoreliving Carcinus moenas.