20 THE PRINCIPLES OF HEREDITY 



to settle it by direct observation. Yet it has great import- 

 ance. If acquirements were shown to be inheritable among 

 these lowly forms, no proof, of course, would be afforded of 

 their inheritability higher in the scale, where the probability 

 of transmission is infinitely less. But, if they were shown not 

 to be transmissible among unicellular forms, an overwhelming 

 presumption would be raised against their transmission among 

 multicellular plants and animals. 



34. It is a well-known fact that diseases are apt to change 

 their type when the environment, in which the pathogenic 

 organisms exists, is changed. Thus small-pox when removed 

 to the calf is altered to cow-pox. It becomes a purely local 

 and contagious, not a general and air-borne malady. The 

 organisms of rabies, passed through a succession of monkeys, 

 grow milder in type ; passed through a succession of dogs, 

 they grow more virulent; passed through a succession of 

 rabbits, still more virulent. The organisms of anthrax may 

 be attenuated by heat ; those of diphtheria and tuberculosis 

 by cultivation in artificial media. Numerous similar examples 

 may be instanced. It is supposed by bacteriologists that all 

 these organisms become individually modified by their altered 

 surroundings, and that their " modifications " are transmitted 

 to descendants. But the belief does not appear to be founded 

 on anything more than an assumption. Unicellular forms, 

 especially the organisms of disease, multiply with extreme 

 rapidity, and therefore are exceptionally fitted to undergo 

 swift racial change through the survival of the fittest 

 through the accentuation of * ' spontaneous " variations. They 



in bi-parental reproduction denying all influence to the environment. 

 But the dissimilarity between germ-plasms must have arisen in some 

 way. He derived it, therefore, from the remote unicellular ancestors of 

 multicellular forms. He supposed that unicellular organisms, among 

 whom, as he then supposed, there was no amphimixis, underwent 

 racial change solely through the transmission of modifications caused by 

 the action of the environment. But amphimixis among infusorians was 

 subsequently proved by Maupas. (Le rejeunissement kalyogamique chez 

 les Oiltis. Arch. d. Zobl., 2 m * se"rie, vii. 1889.) Thereupon Weismann 

 concluded that even these unicellular organisms must have arisen 

 through the Natural Selection of congenital variations, and therefore 

 that the source of variation " must be dismissed to some stage less dis- 

 tant from the origin of life " to " those lowest of all beings which are 

 entirely formless, and have no fixed size, beings which we must regard, 

 little as we know about them, as crossing the very threshold of organic 

 life." (Estays, vol. ii., pp. 193-4). His latest opinion, like that of most 

 biologists, is that the environment, acting on the germ-plasm, may, on 

 rare occasions and to a small extent, cause variations even in the highest 

 multicellular forms. (The Germ-plasm, p. 418.) 



