XX 



complete sterility, as " incidental to other acquired differences." 

 (Ibid., p. 245.) 



Considering the difficulties which surround the question of the 

 causes of variation, it is not to be wondered at, that Darwin should 

 have inclined, sometimes, rather more to one and, sometimes, rather 

 more to another of the possible alternatives. There is little differ- 

 ence between the last edition of the 1 Origin ' (1872) and the first on 

 this head. In 1876, however, he writes to Moritz Wagner, " In my 

 opinion, the greatest error which I have committed has been not 

 allowing sufficient weight to the direct action of the environment, 

 i.e., food, climate, &c, independently of natural selection .... 

 When I wrote the 'Origin,' and for some years afterwards, I could 

 find little good evidence of the direct action of the environment; now 

 there is a large body of evidence, and your case of the Saturnia is 

 one of the most remarkable of which I have heard." (Ill, p. 159.) 

 But there is really nothing to prevent the most tenacious adherent to 

 the theory of natural selection from taking any view he pleases as to 

 the importance of the direct influence of conditions and the here- 

 ditary transmissibility of the modifications which they produce. In 

 fact, there is a good deal to be said for the view that the so-called 

 direct influence of conditions is itself a case of selection. Whether 

 the hypothesis of Pangenesis be accepted or rejected, it can hardly be 

 doubted that the struggle for existence goes on not merely between 

 distinct organisms, but between the physiological units of which 

 each organism is composed, and that changes in external conditions 

 favour some and hinder others. 



After a short stay in Cambridge, Darwin resided in London for 

 the first five years which followed his return to England ; and 

 for three years, he held the post of Secretary to the Geological 

 Society, though he shared to the full his friend Lyell's objection to 

 entanglement in such engagements. In fact, he used to say in later 

 life, more than half in earnest, that he gave up hoping for work from 

 men who accepted official duties and, especially, Government appoint- 

 ments. Happily for him he was exempted from the necessity of making 

 any sacrifice of this kind, but an even heavier burden was laid upon 

 him. During the earlier half of his voyage Darwin retained the 

 vigorous health of his boyhood, and indeed, proved himself to be 

 exceptionally capable of enduring fatigue and privation. An anoma- 

 lous but severe disorder, which laid him up for several weeks at Val- 

 paraiso in 1834, however, seems to have left its mark on his constitu- 

 tion ; and, in the later years of his London life, attacks of illness, 

 usually accompanied by severe vomiting and great prostration of 

 strength, became frequent. As he grew older, a considerable part of 

 every day, even at his best times, was spent in misery ; while, not 



