292 THE WORLD OF LIFE CHAP. 



is so universal a fact that, with most people, it ceases to 

 excite wonder or curiosity. Yet it is to this day absolutely 

 inexplicable. No doubt an immense deal has been discovered 

 of the mechanism of growth, but of the nature of the forces 

 at work, or of the directive agencies that guide and regulate 

 the forces, we have nothing but the vaguest hints and con- 

 jectures. All growth, animal or vegetable, has been long 

 since ascertained to begin with the formation and division 

 of cells. A cell is a minute mass of protoplasm, a substance 

 held to be the physical basis of life. This is, chemically, the 

 most complex substance known, for while it consists mainly 

 of four elements carbon, hydrogen, nitrogen, and oxygen 

 it is now ascertained that eight other elements are 

 always present in cells composed of it sulphur, phosphorus, 

 chlorine, potassium, sodium, magnesium, calcium, and iron. 

 Besides these, six others are occasionally found, but are not 

 essential constituents of protoplasm. These are silicon, 

 fluorine, bromine, iodine, aluminium, and manganese. 1 



Protoplasm is so complex a substance, not only in the 

 number of the elements it contains, but also in the mode of 

 their chemical combination, that it is quite beyond the reach 

 of chemical analysis. It has been divided into three groups 

 of chemical substances proteids, carbohydrates, and fats. 

 The first is always present in cells, and consists of five 

 elements carbon, hydrogen, sulphur, nitrogen, and oxygen. 

 The two other groups of organic bodies, carbohydrates and 

 fats, consist of three elements only carbon, hydrogen, and 

 oxygen, the carbohydrates forming a large proportion of 

 vegetable products, the fats those of animals. These also 

 are highly complex in their chemical structure, but being 

 products rather than the essential substance of living things, 

 they are more amenable to chemical research, and large 

 numbers of them, including vegetable and animal acids, 

 glycerin, grape sugar, indigo, caffeine, and many others, have 

 been produced in the laboratory, but always by the use of 

 other organic products, not from the simple elements used 

 by nature. 



The atomic structure of the proteids is, however, so 

 wonderfully complex as to be almost impossible of deter- 



1 Verworn's General Physiology, p. 100. 



