may vary immensely in shape and size: and Professor Hickson declares (in a paper published while these sheets are passing through the press) that after many years of laborious work in striving to determine species of these animal colonies, he feels ãquite convinced that we have been engaged in a more or less fruitless task23ã.
The formation of a tooth has very lately been shown to be a phenomenon of the same order. That is to say, ãcalcification in both dentine and enamel {425} is in great part a physical phenomenon; the actual deposit in both tissues occurs in the form of calôÙcoôÙspheôÙrites, and the process in mammalian tissue is identical in every point with the same process occurring in lower organisms24.ã The ossification of bone, we may be sure, is in the same sense and to the same extent a physical phenomenon.
The typical structure of a calcospherite is no other than that of a pearl, nor does it differ essentially from that of the otolith of a mollusc or of a bony fish. (The otoliths, by the way, of the elasmobranch fishes, like those of reptiles and birds, are not developed after this fashion, but are true crystals of calc-spar.)
Throughout these phenomena, the effect of surface-tension