There is another interesting fact in connection with Paramœcium. Under natural conditions, division and redivision continue in the ordinary way for a large and indefinite number of generations. But very occasionally, a process known as _conjugation_ occurs. Two individuals lay themselves side by side, and partially unite; they exchange portions of their nuclear substance, and finally separate again, simple division afterwards proceeding as before. Conjugation, although distinctly different from the ordinary process of sexual reproduction, appears to serve the same purpose. Until quite lately its meaning, and that of the process of sexual reproduction in general, seemed to bid fair to remain a perpetual puzzle to biologists. But at last we seem to be approaching the solution. The characters of a species are determined, it is tolerably certain, by the constitution of the cell nucleus, and accordingly as this varies from one individual to another, so the characters of the individuals will vary. Now, if simple division were to continue indefinitely, successive generations would be produced on the same plan, and the racial characters would in the main remain constant. But conditions of life vary from time to time and from place to place, and the particular type which succeeds best under one set of circumstances may be ill adapted for another. It is therefore an advantage to a race to be capable of variation. And the process of sexual reproduction, by continually bringing about a mixture of the nuclear substance, ensures the regular production of a variety of types. Of these various combinations of characters the few that are suited to the prevailing conditions will, for the time being, constitute the dominant types. When conditions change, fresh types will be available to replace them. The process of conjugation is illustrated in Fig. 25. [Illustration: FIG. 25.--Stages in conjugation of Paramœcium. _meg._, The meganucleus; _mic._, the micronucleus, which divides, and half of which is exchanged; _p.b._, Polar bodies, which the micronucleus throws off, and which disappear. From Dendy's _Outlines of Evolutionary Biology_ (Constable).] There are many groups of one-celled animals other than those typified in the Amœba and the Paramœcium, but they do not appear to have any significance so far as the descent of the higher animals is concerned, and they therefore do not immediately concern us.