472 TROPICAL NATURE 



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 Expression 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, 1 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 Insectivorous 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- 



i 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. 



