60 THE EVOLUTION THEORY 



goes beyond the endosperm, and the riddle of this influence is solved 

 in the most unexpected manner, indeed was solved even before Correns 

 had securely established the genuine occurrence of Xenia. The ex- 

 planation is due to recent disclosures in regard to the processes of 

 fertilization in flowering plants. 



It had long been known that the pollen-tube contains not merely 

 one generative nucleus but two, which arise from one by division. 

 But what had till recently remained unknown was that not only one 

 of these penetrated into the embryo-sac to enter into amphimixis with 

 the egg-cell, but that the other also makes its way in, and there fuses 

 with the two nuclei which had long been designated the upper and 

 lower polar-nuclei (Fig. 82, op. cit.). Nawaschin and Guignard 

 demonstrated that these two nuclei fuse tvith the second vnale nucleus; 

 thus two acts of fertilization are accomplished in the embryo-sac, and 

 one of these gives rise to the embryo, while the second becomes 

 nothing less than the endosperm, the nutritive layer which surrounds 

 the embryo, whose origin from the polar nuclei had been previously 

 recognized. 



Thus the riddle of Xenia is essentially solved. We understand 

 how paternal primary constituents may find their way into the 

 endosperm, and indeed must do so regularly; we understand also 

 how the paternal influence never goes beyond the endosperm. The 

 riddle is thus not only solved, but at the same time the view which 

 assumes a fixed germ-plasm, and believes it to lie in the nuclear 

 substance of the germ-cells, receives further confirmation, if it 

 should need any, for if facts which are apparently contradictory to 

 a theory can be naturally brought into harmony with it, this affords 

 a stronger argument for the correctness of the theory than the power 

 of explaining the facts which were used in building it up. 



There is much more to be said in regard to Xenia, and I am 

 sure that much that is of interest will be brought to light by deeper 

 investigation; theoretical difficulties will still have to be overcome, 

 and I have already pointed out one of these in my ' Germ-plasm,' but 

 I must here rest satisfied with what has been already said. 



We have now passed in review and attempted to fit into the 

 theory a sufficiently large number of the phenomena of heredity for 

 the 'purpose of these lectures. Although, as is natural, much of this 

 must remain hypothetical, we may accept the following series of 

 propositions as well founded : there is a hereditary substance, the 

 germ-plasm ; it is contained in very minimal quantity in the germ- 

 cells, and there in the chromosomes of the nucleus; it consists of 

 primary constituents or determinants, which in their diverse arrange- 



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