642 THE POPULAR SCIENCE MONTHLY. 



Following out his teleological argument, Cuvier remarks that the 

 organization of this cavity and its appurtenances must needs vary- 

 according to the nature of the aliment, and the operations which it 

 has to undergo, before it can be converted into substances fitted for 

 absorption ; while the atmosphere and the earth supply plants with 

 juices ready prepared, and which can be absorbed immediately. 



As the animal body required to be indei^endent of heat and of the 

 atmosphere, there were no means by which the motion of its fluids 

 could be produced by internal causes. Hence arose the second great 

 distinctive character of animals, 01 the circulatory system, which is 

 less important than the digestive, since it was unnecessary, and there- 

 fore is absent, in the more simple animals. 



Animals further needed muscles for locomotion and nerves for 

 sensibility. Hence, says Cuvier, it was necessary that the chemical 

 composition of the animal body should be more complicated than that 

 of the plant ; and it is so, inasmuch as an additional substance, nitro- 

 gen, enters into it as an essential element, while in plants nitrogen 

 is only accidentally joined with the three other fundamental constitu- 

 ents of organic beings carbon, hydrogen, and oxygen. Indeed, he 

 afterward affirms that nitrogen is peculiar to animals ; and herein he 

 places the third distinction between the animal and the plant. 



The soil and the atmosphere supply plants with water, composed 

 of hydrogen and oxygen ; air, consisting of nitrogen and oxygen ; and 

 carbonic acid, containing carbon and oxygen. They retain the hydro- 

 gen and the carbon, exhale the superfluous oxygen, and absorb little 

 or no nitrogen. Tlie essential character of vegetable life is the ex- 

 halation of oxygen, which is effected through the agency of light. 



Animals, on the contrary, derive their nourishment either directly 

 or indirectly from plants. They get rid of the superfluous hydrogen 

 and carbon, and accumulate nitrogen. 



The relations of plants and animals to the atmosphere are there- 

 fore 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 dis- 

 tinctive character. 



Thus wa-ote Cuvier in 1828. But, in the fourth and fifth decades 

 of this century, the greatest and most rapid revolution which biologi- 

 cal 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 em- 

 ployment of instruments of precision for the measurement of the physi- 

 cal 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 Charoe^ are in constant and 



