ORGANIC EVOLUTION — THE FACTORS 71 



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 follow- 

 ing. If we pass the virus of small-pox, by means of 

 inoculation, through a series of calves, the disease be- 

 comes 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 

 jlie 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 pro- 

 foundly 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 ii>. 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 Weis- 

 mann — 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 



