10 THE NEWEST THEORIES CONCERNING EVOLUTION sec. 



germ-plasm, is located in tlie cell-nucleus, and possesses, by 

 virtue of its extraordinary complexity of structure, the 

 capacity to develop into a very complex organism." 



According to Weismann's explanation of heredity, therefore, 

 the germ-cells appear no longer as the product of the body, 

 but rather as something in contrast with the totality of the body- 

 cells. " The germ-cells of successive generations are related 

 in the same way as a series of generations of unicellular 

 beinsfs which are derived one from another bv continued 

 division." 



The necessary consequence of such a conception is the 

 proposition that acquired characters are never inherited. If 

 he is right, then no external influences whatever can ever 

 have contributed to the moulding of species. The permanence 

 of forms would be explained by that proposition, but their 

 variability would remain, unless other agencies were brought 

 forward for its explanation, a darker mystery than ever. 



Weismann seeks these agencies in sexual reproduction. 



By the mixture of characters which sexual reproduction 

 brings about, according to him, are supplied at once — and 

 exclusively — the materials for the origin of new species ; 

 among the new forms produced by the mixture, the struggle 

 for existence chooses the fittest for survival and for renewed 

 reproduction.^ 



The great importance of the sexual mixing of characters 

 in modification can be ignored by no one, and was indeed never 

 ignored. But if this were the only cause of modification it 

 would be necessary to assume, supposing the same laws to 

 hold for lower as for higher organisms, that all plants and 

 animals, including the lowest, now reproduce sexually, and 

 always have done so, — an assumption which is known to be 



^ A. Weismann, Bie Bedeutung der geschlechtlichen Fortpflanzung far die 

 Selektionstheorie, Address to the Congress of Naturalists at Strassburg, 1885, 

 and Jena, G. Fischer, 1886. 



