58 THE EVOLUTION TIIEOKV 



point ; ill the iiieantimo wo can only nay that ahvailj' — wen aiiiuiii;' 

 hybrids — many deviations from Mendel's Law have been established, 

 for instance, by Bateson and Saunders (1903). 



Kel'ort' I conclude this lectnix^ I should like to refer brietiy to 

 a phenomenon which Darwin was accjuainted with and souj>'ht to 

 explain through his theory of Panj;enesis, but which at a later date 

 A\as regarded as not sufficiently authenticatt'tl to justify any attempt 

 at a theoretical explanation, since it seemed to contradict all our 

 conceptions of hereditary substance and its operations. I refer to 

 the phenomenon to which the botanists ha\e given the pretty name 

 of Xenia (gut'st-gifts), ami which consists in the fact that in crosses 

 of two ditferent plants the characters of the male may appea.r not 

 only in the young plant but even in the seed, so that a transference 

 of paternal characters seems to take place from the pollen-tube to 

 the mother, to the ' tissue of the maternal o\ ary.' In heads of yellow- 

 grained maize {Zca) it -is said that, after dusting with pollen from 

 a blue-seeded variety, blue seeds appear among the yellow, and 

 similar observations on other cultivated plants lia\e been on reet)rd 

 for more than half a century. Thus dusting the stigma of green 

 varieties of grape with the pollen of a dark blue kind is said 

 frequently to give rise to dark blue fruits. 



Darwin accepted these observations as correct, and endeavoured 

 to explain them as due to a migration of his ' gemmules ' from the 

 fertilized ovum into the surrounding tissue of the mother-plant. 

 His explanation was not correct, we can say with confidence now, 

 but he was right so far, for the phenomena of Xenia do occur; they 

 are not illusory as most modern botanists seem inclined to believe. 

 I myself was at first inclineil to wait for further facts in proof that 

 the phenomena of Xenia reall}' occurred befori' attempting to bring 

 them into harmony with my theory, and this will not be found fault 

 with when it is remembered that these cases of Xenia seem to stand 

 in direct contradiction to the fundamental postulates of the germ- 

 plasm theory. For this depends essentially on a dc^linit^^ stable 

 structure of the germ-substance, which lies within the nucleus in 

 the form of chromosomes, and which cannot pass from one cell to 

 another in any other way than by cell-division and division ot the 

 nucleus; how then could it pass from the fertilized ovum to the cells 

 of the endosperm which do not derive their origin from it at all, 

 but from other cells of the embryo-sac? In point of fact some 

 of my opponents have cited Xenia as an actual refutation of my 

 theory. 



That cases of Xenia really do occur is now established by the 



Digitized by Microsoft® 



