Page:George Archdall Reid 1896 The present evolution of man.djvu/56

44 cence and revitalization," without which death must occur, is utterly disproved, as is also the theory that the purpose of conjugation is to maintain the specific average. For here we have unlimited cell-multiplication without any rejuvenescence by means of conjugation, while the specific average is perfectly maintained by the similarity of the conditions to which the different individuals of the species are exposed. On the other hand, Weismann's contention, that conjugation is not necessarily essential to persistence, but is merely a condition which is usually, but not always, advantageous—so advantageous that in nearly all plants and animals it periodically occurs—is fully borne out.

Death, that is death from internal causes, from failure of the vital powers, not death from external causes, such as cold, hunger, accident, &c., occurs for quite another reason, which may be set forth as follows. In those plants which multiply by means of suckers or are multiplied by means of cuttings, the cells whence the individual is derived are to be regarded as little differentiated and specialized, like the cells of the sponge, cell-differentiation and specialization not having proceeded nearly so far among plants as among animals; for the highest plants are inferior in this respect to animals low in the scale. As a result, while plants high in the scale are able to reproduce otherwise than by means of germ cells, animals comparatively very low in the scale are not so able. They reproduce solely by means of germ cells, for such, owing to specialization in function, is the interdependence of their other cells, that these latter cannot exist apart from one another, any more than can a bricklayer, who does nothing else to support life, exist apart from other men. They must remain part of the organism or perish, while they cannot continue to multiply indefinitely, for then the organism would grow too large for its available supply of nutriment.