SELECTION AND VARIABILITY 



must coiiic to a standstill, or the loss must be made good. Only by 

 the production of new hereditary variation on a vast scale could 

 this come about. It was to the means by which this new variation 

 was produced that Darwin devoted his discussion. He supposed that 

 the effects of domestication in altering plants and animals were 

 direct effects of the changed environment. His difficulties, in fact, 

 ultimately led him to adopt the ancient hypothesis of direct adapta- 

 tion, the Lamarckian inlieritance of acquired characters, which he 

 called pangenesis. 



While Darwin was wrestling with this problem, Mendel had 

 already solved it: or rather had shown that it did not really exist. 

 On the mendelian, or indeed any particulate theory of inheritance, 

 as Fisher has pointed out, crossing does not destroy variation. 

 When a tall pea is crossed with a short one, the parental difference 

 vanishes in the F^. But the disappearance is only temporary: both 

 parental types appear once more, side by side, in the Fg. Whatever 

 differences disappear into a hybrid by crossing, reappear in its 

 progeny by segregation. And since there is no permanent loss of 

 variation by crossing there need be no production of new variation 

 on a correspondingly large scale. Nor is there any large scale pro- 

 duction of new variation, as Johannsen proved when he established 

 his pure lines. Mendel's peas removed the need for postulating 

 the rise of variation on a grand scale : Johannsen's beans showed 

 that it did not occur. Darwin had been misled by not knowing 

 enough about heredity. 



Mendel's experiments thus gave Darwinism the foundation it 

 needed. Later experiments in genetics, by developing its particulate 

 theory in the way we have seen, add to the strength of the joint 

 structure. They show us that the most important changes in 

 conditions, which lead to selective adjustment, are not, as was 

 supposed, changes in the inanimate world: they are changes in 

 heredity. One class of these, which we may mention in passing, 

 are such as occur in other species, especially where the species live 

 in the host-parasite relationship. All organisms are either hosts or 

 parasites or both. Any genetic change in either is liable to affect the 

 other, and a continual succession of mutual adaptations is the result. 

 Each mutation to greater virulence by the parasite leads to seleaion 

 of a mutation for greater resistance in the host. The system is shown 



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