THE LIFE AND WORK OF DARWIN 215 



origin of man and his history." In 1871 the 

 "Descent of Man" was published, and in 1872 

 "The Expression of the Emotions," which was 

 originally intended to be only a chapter in the 

 " Descent of Man." 



II. The Series of Botanical Works. 



These dealt with the development of special 

 questions and problems arising in direct connection 

 with the " Origin of Species." 



In 1862 appeared the "Various Contrivances by 

 which Orchids are Fertilised by Insects," of which 

 Professor Huxley says: "Whether we regard its 

 theoretical significance, the excellence of the obser- 

 vations, and the ingenuity of the reasonings which 

 it records, or the prodigious mass of subsequent 

 investigation of which it has been the parent, it has 

 no superior in point of importance." 



From the first, Darwin was convinced that no 

 theory could be satisfactory which did not explain 

 the way in which mechanisms, involving adaptation 

 of structure and function to the performance of 

 certain operations, had come about. In 1793 

 Sprengel had established the fact that in a large 

 number of cases a flower is a piece of mechanism, 

 the object of which is to convert insect visitors into 

 agents of fertilisation. What Sprengel did not do 

 was to show that plants provided with such flowers 

 gained any advantage thereby. Darwin worked at 

 this subject for many years, from 1839 onwards, and 

 showed cross-fertilisation to be favourable, and in 



