Biology 679 



properties of all protoplasm, and on these evolution has reared 

 its great edifice and brought into being that almost ineredible 

 multiplicity of species (nearly a million are known already) of 

 animals and plants, ranging from a whale to a flea, an oak to a 

 toadstool, a tapeworm to a bird, a bacterium to a lily, a jelly-fish 

 to an ant-community, a worm to a philosopher. 



3 



The Units of Life 



We have already seen that apparently homogeneous bits of 

 matter a half-crown, or a tumbler-full of water, or a grain of 

 salt are all composed of minute material units, or molecules. 

 In a similar way, the bodies of living things are built up out of 

 units. If we dissect the body of a human being or an animal, 

 we find that it is formed of a number of organs such as the heart, 

 the stomach, the brain, the hand, each with some particular type 

 of work to do for the benefit of the whole organism. Every organ 

 in its turn is composed of a number of tissues, each of which seems 

 to be homogeneous. The stomach, for instance, is formed of 

 secreting tissue inside, muscular tissue outside, and it is supported 

 and bound together by connective tissue, and is penetrated 

 throughout by blood-tissue in the blood-vessels and nervous tissue 

 in the nerves. 



But when we come to examine these tissues under a good 

 microscope, we find that they are not homogeneous at all, but com- 

 posed of a number of separate units called cells. In the blood 

 these cells are separate and independent, while in other tissues 

 they are united with each other. 



Many people still find it difficult to believe that man is de- 

 scended from Simian ancestors, these in their turn from lower 

 mammals, and so from ever simpler and simpler progenitors. 

 They can never have grasped the plain fact of observation, that 

 the body of every man and woman alive has grown from a minute 

 undifferentiated cell. 



