One of the most remarkable and interesting sections of Powers' paper is that in which he describes the differences in bodily structure and habits which he attributes to cannibalism, and the whole account of the phenomena should be read in the original. It appears that there are two extremely distinct types of larvae, those with narrow heads and slender bodies which live for the most part on small Crustacea such as _Daphnias_, and those with huge mouths and very wide heads, which disregard such small animals altogether and live on amphibian larvae, whether of their own or other species. As the illustrations show, the differences between these two types are very great, and the differences in instinct and behaviour are no less. The cannibals take no heed of the pelagic crustacea, lying sluggishly at the bottom, rousing themselves immediately to a violent attack on the larger living things which approach them. Nothing but the most incontrovertible evidence based on abundant control experiments should convince us that such differences are not primarily genetic, and in the present state of knowledge I incline to think that the families really consist of individuals which are ready to assume the cannibal habit if opportunity offers, and others which are congenitally incapable of it. It may readily be that if all chance of cannibal diet be excluded, the full development of the wide head and mouth, or the other peculiarities, would never become pronounced, but I doubt whether such change could be induced in any individual taken at random. FOOTNOTES: [1] Pringsheim, H., _Die Variabilität niederer Organismen_, Berlin, 1910. [2] F. Wolf, Modifikationen u. Mutationen von Bakterien, _Zts. F. indukt. Abstam. u. Vererbungslehre_, II, 1909, p. 90. [3] Winslow, C. E. A. and A. R., _Systematic Relationships of the Coccaceae_. New York. 1909. [4] C. C. Dobell, _Jour. Genetics_, 1912, II, p. 201, where full references are given. Still more recently the same author has contributed an excellent summary of the evidence relating to bacteria (_ibid._, II. 1913, p. 325). [5] See Woltereck, _Verh. d. Deut. Zool. Ges._, 1909, p. 110; and 1911, p. 142. This is a subject which can only be properly appreciated on reference to the original papers. Several complications are involved to which I have not here alluded. [6] _Proc. Roy. Soc._, B, Vol. 86, 1913, p. 113. [7] _An Investigation of Evolution in Chrysomelid Beetles of the Genus Leptinotarsa_, Carnegie Publications, 1906, No. 48. [8] This is the famous Colorado beetle or potato-bug, which has caused such serious destruction in potato crops. There seems to be no doubt that this insect, formerly unknown in the eastern States, made its way east along the mining trails when the west was opened up. [9] This is indicated in the coloured plate, but I have not found any explicit statement to this effect in the text, and am not sure if the absence of pigment was regarded as complete.