22 DOCTRINE OF EVOLUTION 



cell proves to be the '^organic individual of the first 

 order." As the ultimate biological unit, its essential 

 nature must possess a profound interest, for in its 

 substance resides the secret of life. 



This wonderful physical basis of life is called proto- 

 plasm. It contains three kinds of chemical compounds 

 known as the proteins, carbohydrates, and hydro- 

 carbons. Proteins are invariably present in living cells, 

 and are made up of carbon, hydrogen, nitrogen, sulphur, 

 and usually a little phosphorus. The elements are 

 also combined in a very complex chemical way. For 

 example, the substance called haemoglobin is the protein 

 which exists in the red blood cells and which causes those 

 cells to appear light red or yellow when seen singly. 

 Its chemical formula states the precise number of atoms 

 which enter into the constitution of a single molecule 

 as : CgQoHggQNig^FeOi^g. This is truly a marvelously 

 complex substance when compared with the materials 

 of the inorganic world, like water, for example, which has 

 the formula HgO. And just as the peculiar properties 

 of HgO are given to it by the properties of the hydrogen 

 and the oxygen which combine to form it, just so, the 

 scientist believes, the marvelous properties of protein 

 are due to the assemblage of the properties of the 

 carbon and hydrogen and other elements which enter 

 into its composition. 



It would be interesting to see how each one of these 

 elements contributes some particular characteristic 

 to the whole compound. The carbon atom, for ex- 

 ample, is prone to combine with other atoms in defi- 

 nite varied ways, and the high degree of complexity 

 which the protein molecule possesses may depend in 



