fact that their inmost convictions on matters of the deepest 
scientific importance are to be read, often in the compass of 
a brief sentence. There we find, as we cannot find in any 
other way, the real core of the matter, with all accessory and 
surrounding considerations stripped away from it.* A care- 
ful study of the two recent volumes of Darwin’s letters, and 
a re-study of the three earlier volumes, with a view to this 
Address, have shown how Darwin’s thoughts were again and 
again occupied upon subjects bound up with the problem I 
have ventured to bring before you this evening. The interest 
reaches its height when we find that strongly-marked differ- 
ences of opinion on fundamental questions are threshed out 
in the correspondence, when we see, as I shall have occasion 
to point out in greater detail in the later pages of this Address, 
Darwin differing sharply from Huxley on the one hand, and 
with Wallace on the other, as to the significance and history 
of sterility between species. 
In such episodes we are permitted to become the witnesses 
of a supremely interesting struggle, where the central figure 
of modern biological inquiry is contending with his chief 
comrades in the great fight,—with the co-discoverer of natural 
selection, with the warrior hero who stood in the forefront of 
the battle. 
The correspondence of Charles Darwin has a further deep 
interest for us. We see the means by which a gentle, 
sympathetic, intensely human nature overpassed the stern 
limits imposed by health, and was able to impart and to 
receive fresh ideas, and a stimulus ever renewed—the im- 
pulse to varied and unceasing research. I have lately been 
studying with keen interest the life of another great English- 
man, William John Burchell,} than whom no better equipped 
or more learned traveller ever explored large areas in two 
continents. When I state that searching inquiry has only 
brought to light a dozen of his letters, and that he was 
known to hardly any of the great naturalists of his day, we 
see the reason for the sad, unproductive, brooding close of a 
career which opened with almost unexampled brilliancy and 
* «Quarterly Review,” January 1901, p. 258. 
+ ‘‘Ann. and Mag. Nat. Hist.,” January 1904, p. 45. 
