is derived, i.e. not a germ cell from the same body, but only with a germ from another body.
Thus these remote cell-descendants of the original pair of cells which conjugated, and from which the body to which they belong is derived, exhibit, after many generations, the same peculiarity which Maupas found distinguished the remote cell-descendants of conjugated unicellular organisms; viz. that after a certain number of cell-generations, unless conjugation occurs anew, they perish, and with them the race. The other cells which do not conjugate cease in time, like Maupas' infusorians, to multiply, and, like those infusorians, perish, and with them the individual of whom they generally form the chief part.
The above seems to bear out the theory that conjugation causes a "rejuvenescence and revitalization" without which the race cannot persist; but on closer examination the facts are found to point the other way. The number of cell-generations following conjugation differs in different individuals of the same species, and may be made to differ in each individual by variations in nutrition, exercise, &c. It differs enormously in different species of plants and animals; thus the number of cell-generations following conjugation is enormously fewer in minute plants and insects than in such great plants and animals as pines and whales. Moreover the number of non-conjugating cell-generations is almost infinitely prolonged along certain lines in such animals as the aphides, which reproduce asexually during the whole summer, and reproduce sexually only on the advent of cold weather, and which, were the warm weather to continue, might reproduce asexually for ever. It is infinitely prolonged in such plants as multiply by means of suckers, or are propagated by cuttings, without their cells ever conjugating. The theory therefore that conjugation causes a "rejuvenes-
