60 THE STORY OF THE LIVING MACHINE. 



how it would be learned that it is the human being 

 alone that grows and multiplies and that all else 

 is the result of his activities. Such a supramun- 

 dane observer would find himself entering into a 

 new era, in which all his previous knowledge would 

 sink into oblivion. 



Something of this same sort of revolution was 

 inaugurated in the study of living things by the 

 discovery of cells and protoplasms. Animals and 

 plants had been studied for centuries and many 

 accurate and painstaking observations had been 

 made upon them. Monumental masses of evi- 

 dence had been collected bearing upon their 

 shapes, sizes, distribution, and relations. Anato- 

 my had long occupied the attention of naturalists, 

 and the general structure of animals and plants 

 was already well known. But the discoveries 

 starting in the fourth decade of the century by 

 disclosing the unity of activity changed the aspect 

 of biological science. 



The Cell Doctrine. The cell doctrine is, in 

 brief, the theory that the bodies of animals and 

 plants are built up entirely of minute elementary 

 units, more or less independent of each other, and 

 all capable of growth and multiplication. This 

 doctrine is commonly regarded as being inau- 

 gurated in 1839 by Schwann. Long before this, 

 however, many microscopists had seen that the 

 bodies of plants are made up of elementary units. 

 In describing the bark of a tree in 1665, Robert 

 Hooke had stated that it was composed of little 

 boxes or cells, and regarded it as a sort of honey- 

 comb structure with its cells filled with air. The 

 term cell quite aptly describes the compartments 

 of such a structure, as can be seen by a glance at 

 Fig. 7, and this term has been retained even till 



