472 TROPICAL NATURE Ix 
Darwin had collected have never been made known. The 
cause is well known to have been the continued pressure of 
ill-health. The work on Domesticated Animals was thus 
delayed many years, after which came the labour of bringing 
out a much enlarged edition of the Origin of Species. The 
Descent of Man was, apparently, at first intended to be a 
comparatively small book, but a difficulty connected with the 
origin of the distinctive peculiarities of the two sexes led to 
an investigation of this subject throughout the animal king- 
dom. This was found to be of such extreme interest, and to 
have such important applications, that its development with 
the completeness characteristic of all the writer's work led to 
the production of two bulky volumes, followed by another 
volume on the Laupression of the Emotions in Man and Animals, 
not less instructive. None of Darwin’s works has excited 
greater interest or more bitter controversy than that on man; 
and the correction of the numerous reprints, and of a final 
enlarged edition in 1874, was found to be so laborious a task 
as to convince him that any such extensive literary works as 
those projected and announced six years previously must be 
finally abandoned. This, however, by no means implied 
cessation from work. Observation and experiment were the 
delight and relaxation of Darwin’s life,t and he now con- 
tinued and supplemented those numerous researches on plants 
we have already referred to. A new edition of an earlier 
work on the Movements of Climbing Plants appeared in 1875 ; 
a thick volume on JInsectivorous Plants in the same year ; 
Cross and Self-Fertilisation in 1876; the Forms of Flowers in 
1877; the Movements of Plants, embodying much original 
research, in 1880; and his remarkable little book on Earth- 
worms in 1881. This last work is highly characteristic of 
the author. In 1837 he had contributed to the Geological 
Society a short paper on the formation of vegetable mould 
by the agency of worms. For more than forty years this 
subject of his early studies was kept in view; experiments 
were made, in one case involving the keeping a field untouched 
for thirty years,—and every opportunity was taken of collect- 
1 About this time he said to the present writer: “When I am obliged to 
give up observation and experiment, I shall die.” And he actually did con- 
tinue his experiments to within a few days of his death, 
