It (the organism) is during life subjected to all manner of external conditions, which tend to destroy it, or so to maim and enfeeble it that it is easily destroyed; for, unlike plants and low animal organisms, the interdependence of the parts as well as of the cells of a higher animal organism is so great, that no one part can be injured without all the other parts being injuriously affected. The ultimate death of all organisms with highly specialized cells is therefore inevitable. To put it another way: all cells or aggregate of cells, which are so specialized as to be incapable of independent existence, cannot continue the race, and must inevitably perish. In high animal organisms the germ cells alone are capable of existing independently, and therefore they alone survive in their descendants. It is true that in the highest animals the germ cells after conjugating are still retained within the body of, and supplied with the nutriment by, one of the parents till development is considerably advanced, but in the sense in which I write they as truly lead a separate existence as the child whom the mother suckles.
But though conjugation is not universally necessary to unending reproduction it is yet generally true; and the point I wish to emphasize is this—that in the highest animals, as well as in the lowly infusorians, unless conjugation occurs after a certain number of cell-generations, cell-multiplication ceases, and the race perishes.
The germ cell, like the unicellular organism, on conjugating, divides and redivides many times without conjugation ever occurring again among its descendant cells, the successive generations of which may be compared to successive non-conjugating generations of infusorians, so that the body of a multicellular organism in the successive stages of its ontogeny is comparable to later and later generations of infusorians; but, unlike
