ABSENCE from England has prevented my taking part in the discussion on this subject. At this stage I only desire to say that I quite agree with Prof. Ray Lankester, as I stated at the meeting of the Linnean Society, with respect to the contention that the specific characters of the systematist are not necessarily those upon which natural selection has directly acted in bringing about the specific differentiation. These external visible or measurable characters may be, and I believe often are, the outward expression of internal differences of constitution with which the external characters are correlated. In entering the lists at this late period, I am, however, mainly prompted by an omission on the part of Prof. Weldon to strengthen his own case by an argument which appears to me to be quite legitimate. The point at issue is whether the results of his laborious and, in my opinion, most valuable measurements of crabs, are to be interpreted as demonstrating the action of selection, or simply as revealing a law of growth. It might be imagined that if the latter alternative proved to be the correct interpretation, the case for selection falls to the ground. I do not take this view of the work, and, as a member of the Royal Society Committee concerned with the investigation, I am glad of the opportunity, afforded by the discussion in these columns, of giving expression to the idea which I have always entertained on this point, for whatever that idea may be worth. If future observation should show that there is no selection at work upon the young stages, weeding out the individuals whose breadth of carapace falls below a certain standard, but (according to the alternative) that the individuals get broader as they grow older, then it appears to me that the measurements may still be interpreted as indicating the action of selection; only the selection would have done its work in the past history of the species, instead of acting now, as on the original assumption. In other words, breadth of carapace (or some character correlated with it) had a selection value in the phylogeny; now this character appears at a late stage in the ontogeny. It is for Prof. Weldon to decide, by further observation, which of these interpretations is to be accepted.