MODERNBIOLOGY 5 65 



various ways to procure proofs for his argument. He bred large quantities 

 of rats, whose tails he cut off at birth, but he never succeeded in finding a 

 rat born tailless, nor did other malformations brought about by outward 

 interference ever reproduce themselves. He therefore felt fully justified in 

 maintaining his standpoint that all changes in the outward appearance of 

 the individual compared with other individuals are due to changes in the 

 germinal plasm; that every so-called acquired character is really produced 

 by a change in the germinal plasm, whereby the body becomes capable of 

 adapting itself to the different external conditions of life. But how, then, 

 have the various life-forms arisen in the course of ages? By means of natural 

 selection, answers Weismann, and by that means alone. The variations that 

 are brought about especially through the amphimixis of sexual reproduction, 

 but also through other changes in the germinal plasm, are advanced or re- 

 tarded by natural selection and thus give rise to new forms, whose germinal 

 plasm is better adapted to the conditions of existence than that of the old 

 forms. Natural selection is thus the cause of the evolution of animate beings, 

 Weismann rejecting Nageli's assumption of internal causes of evolution in- 

 herent in the organisms themselves, for such a theory "cannot explain the 

 finality of the organisms. And yet this is the very riddle that the organic 

 world gives us to solve." Numberless instances are quoted of this adaptabil- 

 ity, this connexion between form and function, and every instance is likewise 

 made to serve as evidence of the creative power of natural selection. 



' ' The continuity of the germinal plasm ' ' and ' ' the omnipotence of natu- 

 ral selection" are two phrases in which Weismann's theory of life used to 

 be summed up. As a result of the former of these ideas — that of the germinal 

 plasm as the preserver of heredity — Weismann has reached by way of specu- 

 lation conclusions to a certain extent foreshadowing those that modern 

 heredity-research has since arrived at by means of exact observation. His 

 subsequent attempts to expand this theory gave him similarly happy in- 

 spirations, as when he localizes the germinal plasm — that is, the preserver 

 of heredity — in the chromosomes of the sexual cells. "The idea in itself 

 was sound," says Johannsen in this connexion. Even Weismann, however, 

 succumbed to the danger of basing his conception of a phenomenon on mere 

 speculation; in his continued efforts to extend his germinal-plasm theory 

 downwards he works out a highly complicated plan to show the structural 

 nature of living substance; every one of its minutest entities consists of a 

 mass of chemical molecules; they are termed "biophores," and he assures 

 us that they are not hypothetical: "They must exist, for the phenomena of 

 life must be bound to an entity of matter." Of biophores are composed the 

 determinants: those units in the germinal plasm that govern the various 

 qualities in the smallest parts of the individual; the determinants in their 

 turn build up the ids, which form larger groups of qualities, and these again 



