14 HOW ANIMALS DEVELOP 



we are so impressed at the way in which all the 

 separate processes work together harmoniously that 

 we call animals "organisms." The processes which 

 keep an animal alive have to be quite as highly 

 organized as the operations in the most complicated 

 mass-production factory. If there is a "secret of life," 

 it is here we must look for it, among the causes 

 which bring about the arrangement of innumerable 

 separate processes into a single harmonious living 

 organism. 



When a numerous and varied set of processes is 

 to be organized it is obviously convenient, and 

 often absolutely necessary, to separate the different 

 jobs among different pieces of apparatus, each of 

 which specializes in carrying out one particular 

 function. Thus a motor-car has a separate apparatus 

 — the carburettor — ^to vaporize the fuel, another 

 apparatus — the dynamo — ^to provide electric power, 

 still another — the sparking plug — to make a spark, 

 and so on. We find the same sort of plan adopted 

 in all animals which attain more than a very minute 

 size. For instance, every living creature has to 

 arrange to absorb oxygen from its surroundings and 

 to transport it in the right quantities to the cells in 

 the body which need it. We find that there are 

 special organs for absorbing it, lungs in animals 

 which breathe air, gills in animals which absorb the 

 oxygen dissolved in water; special organs, the 

 blood-vessels, for transporting the oxygen all through 

 the body after it has been absorbed and dissolved 

 in the blood ; a heart to pump the blood along ; and 



