300 THE HISTORY AND SCOPE OF ZOOLOGY IX 



plished for Zoology its final and most important 

 service. N The earlier half of the nineteenth century is 

 remarkable for the rise, growth, and full development 

 of a new current of thought in relation to living 

 things, expressed in the various doctrines of develop- 

 ment which were promulgated, whether in relation to 

 the origin of individual animals and plants or in rela- 

 tion to their origin from predecessors in past ages. 

 The perfecting of the microscope led to a full compre- 

 hension of the great doctrine of cell-structure and the 

 establishment of the facts (1) that all organisms are 

 either single corpuscles (so-called cells) of living 

 material (microscopic animalcules, etc.) or are built up 

 of an immense number of such units ; (2) that all 

 organisms begin their individual existence as a single 

 unit or corpuscle of living substance, which multiplies 

 by binary fission, the products growing in size and 

 multiplying similarly by binary fission ; and (3) that 

 the life of a multicellular organism is the sum of the 

 activities of the corpuscular units of which it consists, 

 and that the processes of life must be studied in and 

 their explanation obtained from an understanding of 

 the chemical and physical changes which go on in each 

 individual corpuscle or unit of living material or pro- 

 toplasm (cell-theory of Schwann). 



On the other hand, the astronomical theories of 

 development of the solar system from a gaseous con- 

 dition to its present form, put forward by Kant and 

 by Laplace, had impressed men's minds with the con- 

 ception of a general movement of spontaneous progress 

 or development in all nature ; and, though such ideas 



