The process of multiplication is by “fission,” or the division of each Paramecium into two similar animalcules. Not infrequently, however, two Paramecia may be seen to approach each other and come together, funnel to funnel; and in each the nuclei undergo curious changes. The macronucleus breaks up, and is scattered. The micronucleus in each divides into four portions, of which three break up and disappear; while the fourth again divides into two parts, one to be retained and the other to be exchanged for the similar micronuclear product of the other Paramecium. The retained portion and that received in exchange then unite to form a new micronucleus. M. Maupas concludes from his careful observations that, in the absence of such “conjugation” in the mid-period of life, Paramecia pass into a state of senility which ends in decrepitude and death. If this be so, conjugation is in them necessary for the continuance of a healthy race. Here we have what a zoologist would describe as a specialized mode of behaviour of the nuclei; and we have also the behaviour of the minute creatures (which contain the nuclei) as they approach each other and come together in conjugation. Can one wonder that the latter, at any rate, has been regarded as an example of conscious procedure? In truth we do not know in what manner and by what subtle influences the Paramecia are drawn together in conjugation. But it is scarcely logical to base on such ignorance any positive assertion as to conscious attraction. It is better to confess that here is a piece of organic behaviour, the exact conditions of which are at present unexplained. We may take from the writings[1] of Dr. H. S. Jennings, of Harvard, some account of other modes of behaviour among Paramecia. They largely feed upon clotted masses of bacteria. If a number are placed upon a glass slip, together with a small bacterial clot, they will be seen to congregate around the clot and to feed upon it. All apparently press in so as to reach it, or get as near it as possible. And if a number be placed on another slide without any clot, they soon collect in groups in one or more regions, as in Fig. 2, III. It appears as if they were actuated by some social impulse leading them to crowd together and shun isolated positions. Nay, more; it seems as if, after thus collecting and crowding in to some centre of interest, the attractive influence gradually waned; the group spreads, and the Paramecia are less densely packed; the assembly scatters more and more, but still seems to be retained by an invisible boundary beyond which the little creatures do not pass. [Illustration: FIG. 2.--Behaviour of Paramecia (after Jennings).]