DARWIN. liEFORE AND AFTER 171 



the assumption of <»i'eat complexity of structure of the 

 germ plasm, which, as already stated, he identified with 

 the chromatin substance of the nucleus of the ji^erm cells. 

 In other words, inherited characters were supposed to 

 be transmitted from gernl cell to germ cell, and not 

 through the body cells, thus denying the transmission of 

 so-called acquired characters. According to Weismann, 

 an organism can only inherit such characters as were 

 inborn in the parent, and every individual contains in 

 its sexual cells some germ plasm not only of its parents, 

 but of its grandparents and of all its ancestors. Weis- 

 mann's views are set forth in hi^ books, '' The Germ 

 Plasm: a theory of Heredit}," translated in 1893. and 

 his ^'Evolution Theory," published in 1902 and trans- 

 lated into English in 1904. 



The work of Mendel (1822-1884) sheds important light 

 on problems of heredity from another point of view, 

 namely, that of hybridisation. His classical paper on 

 " Experiments in Plant Hybridisation " was published 

 in Briinn in 1865, but was overlooked until about 1900. 

 Mendel worked chiefly on crossing different varieties of 

 the garden pea, Pisum sativum. He found that of certain 

 contrasted couples of parental characters, which did not 

 blend, one was dominant over the other, which w^as 

 latent or recessive. The first generation of hybrids was 

 apparently all dominant, but it was subsequently found 

 that they were really impure dominants. When these 

 hybrids were inbred, it was found that one-quarter of 

 them reverted to the dominant type, one-quarter to the 

 recessive type, while one-half reproduced hybrid features, 

 and that these proportions were maintained when the 

 impure dominants were again inbred. The inbred off- 

 spring of pure dominants and pure recessives bred true. 



Mendel's results indicate that small individual charac 

 ters may occur separately in the germ cells, and may be 

 transmitted separately to the offspring. The individual 

 organism may, perhaps, be composed of a number of 



