424 ELEMENTS OF BIOLOGY 



the essential basis of the modern cell theory. This stimulated an 

 intensive study o£ the physical and chemical nature of protoplasm, 

 but it was not until the final years of the nineteenth century that 

 it was established, largely through the work of an American, E. B. 

 Wilson, and a German, Fischer, that protoplasm is a colloid. Col- 

 loids had previously been described by the chemist, Graham, in 

 1861; it followed inevitably that the studies of the physical and 

 chemical properties of protoplasm during the current century have 

 become special phases of colloid chemistry. The cell doctrine has 

 not gone unaltered since its first promulgation. The discovery that 

 some living agents of disease are so minute as to be filterable 

 (filterable viruses, p. 85) has led to questions as to whether or not 

 the conventional definition of a cell is generally applicable to all 

 living material. Moreover, the development of knowledge of the 

 role played by the endocrine glands and their hormones (p. 203), 

 and the knowledge of the regulation of form development by the 

 relation of dominant and subordinate regions in the embryo and in 

 regeneration have served to emphasize the fact that in Metazoa at 

 least a cell is not an independent unit. What such cells do is con- 

 trolled by other cells, either by means of transported chemical 

 agents or by transmitted influences. 



Division of Biology. The ever-increasing accumulation of 

 facts and the ever-widening fields of interest during the latter half 

 of the nineteenth century accelerated the break-up of the science of 

 Biology into a great number of daughter sciences. Thus Pasteur, a 

 Frenchman, Koch, a German, and Lister, an Englishman, in the 

 latter part of the century, by experimental proof of the bacterial 

 nature of many diseases, established the science of Bacteriology. 

 Modern Embryology as a separate branch of Biology dates from a 

 treatise on Comparative Embryology by Balfour, an Englishman, 

 in 1880. Mendel, Weismann, De Vries, Bateson, and others whose 

 names are associated with the development of the science of Genet- 

 ics, realized their successes during the latter half of the century. 



