This is secured in a different way in Crustaceans which have grown to maturity in a sedentary state, like the Cirrhipeds. These animals, known as 'acorn-shells' and 'barnacles,' are sedentary, sometimes on rocks and stones, sometimes on a movable object, the keel of a ship, floating pieces of wood, cork, or cane, or sometimes attached to turtles or whales, and although they generally occur in great numbers together, they are probably only able to fertilize each other occasionally, and are therefore essentially dependent upon self-fertilization. But Charles Darwin discovered long ago that many of them, notwithstanding their hermaphroditism, have males which are small, dwarf-like, and very mobile organisms, destined only for a very brief life. These seemed quite superfluous in association with hermaphrodite animals, and they have therefore long been regarded as vestigial males, as the last remnant, so to speak, of a past stage of the modern Cirrhipeds, in which the sexes were separate. It is obvious, however, that we must now attribute to them a deeper significance, for these so-called 'primordial males,' although extremely transitory creatures without mouth or intestine, represent a means of securing the cross-fertilization of the species. What importance nature attaches to their preservation is shown especially by the parasitic Cirrhipeds which have been so carefully studied by Fritz Müller and Yves Delage--those sac-like Rhizocephalidæ or root-crustaceans which are altogether disfigured by parasitism. The fully developed animals are hermaphrodite and live partly in, partly upon crabs and hermit-crabs (Fig. 112, _C_, _Sacc_). These hermaphrodites indeed fertilize themselves, but in their youth they are of distinct sexes, and the females are so constituted that they lay eggs for the first time just when the males of the current year are appearing. Thus the first batch of eggs liberated by the females are fertilized by the minute free-swimming 'primordial males,' but after that the females themselves develop testes, and then fertilize themselves; the males die very soon after copulation, and only appear the following year in a new generation. They are therefore far from being mere historic reminiscences, vestiges of the early history of the modern species, for they are the instruments of a regular cross-fertilization of the species, and therefore of a constant mingling of new ids in the germ-plasm. This is not the place to discuss the marvellous life-history of these parasites in detail; I can only say that when we inquire into the whole story, and appreciate the difficulties associated with the persistence of these 'primordial males,' we can no longer doubt that crossing is an indispensable feature of amphimixis--a feature which must at least occasionally occur if amphimixis is to retain its significance. This is shown, it seems to me, especially by these numerous instances of what we may call compulsory retention of