proliferate into an organism different from what would otherwise have arisen, yet it is impossible to believe that the variation in the new organism will, except as a mere fortuitous coincidence, be of the same kind as the variation in the parent organism.
That external causes do alter the constitution of the germ cells is indeed proved by such facts as the following. If we pass the virus of small-pox, by means of inoculation, through a series of calves, the disease becomes altered in character, becomes cow-pox, not small-pox, and if a man be inoculated from the last of the series he also takes cow-pox, as do a succession of human beings inoculated the one from the other with the altered virus. Now since there can be no doubt that small-pox is caused by a microbe, it is clear that residence in the calf during some generations so profoundly alters the nature of this microbe, that it causes in man a disease quite different from that which its ancestors caused. That is to say, in this case acquired variations are transmitted. The explanation is, that every microbe, being a unicellular organism, is a germ cell, and a germ cell, moreover, which, besides being extremely minute, and relatively simple, is one on which external conditions act directly, and which is therefore comparatively easily altered in constitution, an alteration which it transmits to its offspring. This explanation suffices also to explain why bacteriologists are able to produce what are called attenuated cultivations of various microbes.
Our knowledge of the subject is as yet slight, but it seems possible or probable that just as each multicellular organism is composed of many cells, so each cell may be composed of many smaller units—the biaphors of Weismann—which stand to the cell in the same relation as the cell stands to the multicellular organism; and further, just as the cells of a multicellular organism are
