The "reproduction," or producing of new individuals, of many animals and plants can be, and is, effected by the detachment of large pieces of a parent organism. Thus plants split into two or more pieces, each of which carries on life as a new individual. Many worms and polyps multiply by breaking into two or more pieces, and very often the broken-off pieces which thus become new individuals and carry on the race are extremely small, even microscopic in size. The spores of ferns and the minute separable buds of many plants and animals are of this nature. They grow into new individuals without any fusion with fertilizing particles from another individual. Yet there seems to be even in the very simplest living things a need to be met, an advantage to be gained, in the fusion of the substance of two distinct parents in order to carry on the race with the best chance of success. We find that those organisms which can multiply by buds and fission yet also multiply regularly by ovules fertilized by sperms. We see this process in its simplest condition in microscopic plants and animals which are so minute that they consist of only a single "cell"--a single nucleated particle of protoplasm. Such unicellular organisms have definite shape, even limb-like locomotor organs, shells, contractile heart-like cavities within the protoplasm, even mouths, digestive tract, and a vent. They produce new individuals by merely dividing into two equal halves or by more rapidly dividing into several individuals each like the parent, only smaller. But from time to time, at recurring periods or seasons, two of these unicellular individuals (of course, two of the same kind or species) come into contact with one another, not by mere chance, but attracted and impelled (probably by chemical guiding or alluring substances of the nature of perfumes) towards one another, and then fuse into one. Two (or sometimes several) individuals thus melt together and become one individual--a process the exact reverse of the division of one into two. This is known to microscopists as "conjugation." The new individual resulting from conjugation after a time divides, and the individuals thus produced, each consisting of a mixture of the fused and thoroughly mixed substance of the two conjugated individuals, feed and grow and divide in their turn, and so on for several generations, until again the epidemic of conjugation sets in, and the scattered offspring of many distinct pairs of the previous conjugation-season in their turn conjugate.