VI ANIMALS AND PLANTS 165 



the superfluous hydrogen and carbon, and accumu- 

 late nitrogen. The relations of plants and 

 animals to the atmosphere are therefore inverse. 

 The plant withdraws water and carbonic acid from 

 the atmosphere, the animal contributes both to it. 

 Respiration that is, the absorption of oxygen and 

 the exhalation of carbonic acid is the specially 

 animal function of animals, and constitutes their 

 fourth distinctive character. 



Thus wrote Cuvier in 1828. But, in the fourth 

 and fifth decades of this century, the greatest and 

 most rapid revolution which biological science has 

 ever undergone was effected by the application of 

 the modern microscope to the investigation of 

 organic structure ; by the introduction of exact 

 and easily manageable methods of conducting the 

 chemical analysis of organic compounds; and 

 finally, by the employment of instruments of pre- 

 cision for the measurement of the physical forces 

 which are at work in the living economy. 



That the semi-fluid contents (which we now 

 term protoplasm) of the cells of certain plants, 

 such as the Charm, are in constant and regular 

 motion, was made out by Bonaventura Corti a 

 century ago ; but the fact, important as it was, 

 fell into oblivion, and had to be rediscovered by 

 Treviranus in 1807. Robert Brown noted the 

 more complex motions of the protoplasm in the 

 cells of Tradescantia in 1831 ; and now such move- 

 ments of the living substance of plants are well 



