2 AN INTRODUCTION TO THE STUDY OF FOSSILS 



white of an egg is almost pure proteid plus much water.) 

 (2) Physiologically it has the power of waste and repair, of 

 growth and of reproduction. 



Living organisms waste away by oxidation, a kind of internal 

 combustion, but continually repair this waste by additions 

 between the existing molecules. If this addition is greater than 

 that necessary for repair, growth occurs. (Inorganic substances, 

 on the other hand, such as crystals, grow solely by external 

 addition.) Living bodies likewise detach portions of them- 

 selves, which thus acquire independent existence and develop 

 into the form of the parent. 



PLANTS AND ANIMALS DISTINGUISHED 



Plants and animals differ fundamentally in their food ; the 

 former usually feed upon inorganic, and the latter upon organic 

 matter. As a result of this, plants tend to a life of immobility, 

 deriving their food from the soil, water and air about them. 

 They accomplish this through the agency of the sun's rays, 

 which, working through the green coloring matter, — chloro- 

 phyl, combine the unorganized elements into the complex 

 products, — proteids, carbohydrates and fats. Animals, on 

 the other hand, as a result of searching for their food, tend to a 

 life of mobility, deriving their food from plants or other ani- 

 mals ; hence chlorophyl is absent, and as a consequence of its 

 absence starch is not manufactured and thus cellulose is like- 

 wise wanting. (All animals have freedom of movement at 

 some period of their lives, even those sessile w^hen adult, as the 

 sponge, coral and barnacle, can move from place to place in 

 their embryonic life.) These differences between plants and 

 animals are, however, not absolute ; some of the lower forms of 

 each sub-kingdom have properties which in their full develop- 

 ment characterize the opposite sub-kingdom. For example, 

 among plants, fungi live upon organic matter, many algae, as 

 diatoms, spores of cryptogams and the male sexual element of 



