4 INTRODUCTION 



produce certain chemical compounds never formed by lifeless 

 matter, (2) its method of growth, and (3) its power of repro- 

 duction. 



In its chemical composition it invariably contains some form 

 of proteid ; the proteids are complex combinations of carbon, 

 oxygen, hydrogen, nitrogen, and sulphur. They vary consid- 

 erably in their relative composition, but since they occur as a 

 constant constituent of living matter and are formed only by 

 living matter, they afford a chemical characteristic for all kinds 

 of organisms. 



Growth, or an increase in size, is a characteristic of all 

 organisms, whether plant or animal, and of some inorganic 

 forms of matter as well, such as crystals and stalactites. But 

 the growth of these lifeless bodies is brought about merely 

 by accretion, the addition of particles to the outer surface. 

 Growth in living bodies, on the other hand, is due to a process 

 of intussusception, an interposition of particles amongst those 

 which already exist ; it is thus a growth within the mass. 



In the third place, living matter is capable of giving off 

 portions of its own substance which may grow into complete 

 individuals ; in other words, it has the power of reproducing 

 itself, a phenomenon never exhibited in lifeless matter. 



When we come to study the finer structure of living bodies 

 we find that they consist of a translucent, semifluid substance, 

 practically colorless and very finely granular. This substance 

 is called protoplasm and is sometimes spoken of as the physi- 

 cal basis of life. In its active state it contains a large amount 

 of water, but its exact structure and composition are still in 

 dispute. It is a very complex combination of carbon, oxygen, 

 hvdrogen, nitrogen, and sulphur, and often contains phosphorus 

 as well ; the sulphur and the phosphorus are in very small 

 quantities. We usually find, more or less intimately associated 

 with its proteid constituents, certain carbohydrates, compounds 

 of carbon, hydrogen, and oxygen, of which the most familiar 

 examples are starch (C 6 H 10 O 5 ) and the various sugars; fats, 

 which contain relatively much less oxygen than the carbo- 

 hydrates, are also commonly found in protoplasm. It has not 

 been possible to obtain a definite chemical symbol for proto- 

 plasm ; whether its elements are combined in fairly definite 



