250 SCIENCE PROGRESS 



sketch of reality ; in all probability it is erroneous in some re- 

 spects. But it is at least the only hypothesis which allows 

 us to synthesise so many distinct sets of facts in one conception. 

 The Mendelian doctrines of unit-factors and segregation ; 

 the cytology of the chromosomes, their constancy of number, 

 their individuality, their mitosis and reduction ; the phenomena 

 of linkage and crossing-over ; the continuity of the germ-plasm ; 

 mutation ; the variability of species on the one hand, their rela- 

 tive constancy on the other ; the function of sexual fusion ; the 

 epigenetic course of development and the restoration of organic 

 form-equilibrium ; the hereditary effects observed by Guyer 

 after injection of anti-lens serum ; sex-linkage and the basis of 

 sex-inheritance ; the abnormalities of non-disjunction and of 

 duplication, — all these can be blended harmoniously in the 

 hypothesis which sees in the chromosomes the bearers of the 

 hereditary constitution of organisms under the form of a 

 definite structural complex of self-reproducing genes. 



