654 LECTURE XXIII. 



is, that by the fusion of two incomplete sexual reproductive 

 cells, a complete reproductive cell, a spore, is formed. The 

 throwing off of the polar bodies indicates the impoverishment, 

 as it were, of the sexual reproductive cells, which presents 

 parthenogenesis, and renders cross-fertilisation, with all its 

 attendant advantages, a possibility. Further, it affords an 

 explanation of heredity, crossing, and hybridisation. A new 

 individual, whether produced by vegetative reproduction or 

 by spores, will more or less closely resemble its parent or 

 parents, because it contains gemmules derived from all parts 

 of the parent or parents. When a cross or hybrid is inter- 

 mediate in character between its parents, it is so, as we have 

 seen, because the sexual cells agree in power ; we restate this, 

 according to the theory of Pangenesis, by saying that each 

 parent contributes an equal number of equivalent gemmules to 

 the offspring. When, on the other hand, a cross or a hybrid 

 resembles one parent more than the other, when the one 

 parent is prepotent over the other, it is because the gemmules 

 derived from the prepotent parent have some advantage in 

 number, affinity, or vigour, over those derived from the other 

 parent. Reversion is explained by the assumption that the 

 gemmules remain dormant through several generations, and 

 when they proceed to develope in any individual it manifests 

 the characters of the individual from which the gemmules 

 were originally derived. This property of the gemmules to 

 lie dormant may also be used to explain alternation of gene- 

 rations. In plants exhibiting this it is, as we have seen, the 

 alternate generations which resemble each other ; sporophore 

 resembles sporophore, and oophore resembles oophore. The 

 gemmules derived from a sporophore lie dormant in the oo- 

 phore, and develope in the succeeding sporophore ; similarly, 

 the gemmules derived from an oophore are dormant in the 

 sporophore and develope in the succeeding oophore. Finally, 

 the increased variability which is induced by changed condi- 

 tions, cultivation for instance, is ascribed to an influence on 

 the reproductive organs which leads to an irregular aggrega- 

 tion of the gemmules in them, some being in excess and 

 others deficient. As to the variation which results from the 



