It is clear that the tendency of this process is to prevent the continued multiplication of one stock or line of descent in a pure state. By conjugation different lines of descent--the progeny of different individuals, often brought together from widely separate localities--are blended and fused. And this is, we are led to conclude, a matter of immense importance. To effect this mixture of separate stocks is, as Darwin has shown, a prime purpose of the habits and structures implanted in the very substance of living things, and developed and accentuated in endless ways and with extraordinary elaboration of mechanisms and procedure during the immense lapse of ages during which life has unfolded and developed on this earth. The fusion of different strains by conjugation gives increased variation in the offspring or new generations: for the two parental strains differ more or less, as all living individuals do, from one another. The result of their fusion is different from either parent. In fact, the process of fusion itself causes a disturbance--a readjustment of the living matter--so that completely new variations result and are selected or rejected in the struggle for existence. Either parental strain was perhaps not so suitable to a newly developed change in the surrounding conditions of life as the new blend may be. Thus a more certain and active production of possibly useful variations is provided for than would be the case were the variations of one self-multiplying stock alone presented for selection.
In the case of simple conjugation the cell individuals which fuse or "mate" with one another, and may be called "maters" or "mating cells," are in all respects similar to one another. But we find among the unicellular plants and animals cases in which one of the mating cells, instead of fusing with another straight away, divides into a number of much smaller cells, which are very active in locomotion and are specially produced in order to mate or fuse with the larger cells. The mating cells are called "gametes," and the large motionless mating cells are called "macro-gametes," or "large maters," whilst the small motile mating cells are called "micro-gametes," or small "maters." The former are of the same nature as egg cells or ovules, the female reproductive particles, whilst the latter, the small "maters," are identical in nature with the sperms or spermatozoa or male reproductive particles of higher organisms. In the case of certain parasitic unicellular animals called coccidia, and also in the parasite which causes malarial fever, quantities of small "mating cells" are produced which fuse with or "fertilize" other much larger mating cells. The small "maters" of coccidia have long vibrating tails and minute oblong bodies, and agree closely in appearance and active locomotion with the spermatozoa of higher animals and plants. The large spherical mating cells might be mistaken for the egg cells of larger animals. In the globe animalcule, Volvox globator, we find a transitional condition leading us to the production of small (male) and large (female) mating cells, like those regularly produced by the massive plants and animals which are built up by hundreds of thousands of "cells" or protoplasmic units conjoined and performing different services for the common life. Volvox is one of those simple aquatic organisms which is not a single cell but a group of many cells (some hundred) hanging together--in this case so as to form a hollow sphere. All the cells of an individual sphere are alike, and have originated by division from one first cell. When the "breeding season" arrives one or two cells of the sphere increase in bulk--they become "large mating cells"--in fact, egg cells. At the same time one or two divide (without separating), so as to form packets of minute oblong cells with vibrating tails. These are "small maters," or "spermatozoa." When ripe they separate and swim away to fertilize--that is to say, to fuse with--the large "mating cells" or egg cells of other Volvox spheres. Such a Volvox sphere as I have described is "bi-sexual": it produces both large and small mating cells, both male and female reproductive cells. But sometimes we find that a number of Volvox spheres produce only large mating cells by the swelling up of one or two of their constituent cells. They are, in fact, female Volvox spheres. And other Volvox spheres produce only packets of small mating cells by the splitting and change of one or two of their constituent cells. They are male Volvox spheres.