30 BIOLOGY 



it does by acquiring amoeboid cells which we call the 

 white blood corpuscles. This fluid or blood becomes 

 more dense, and consequently its oxygen-affinity in- 

 creases. This difference continues to increase until we 

 find a second type of corpuscles evolved, the red ; and 

 the proportion of red to white increases, along with the 

 development of organs which are adapted to increase 

 the supply of oxygen required by this fluid to do its 

 work. 



Simultaneously with the development of these oxygen- 

 supplying organs, whether they be gills as in fishes or 

 lungs as in other forms, we find a gradually increasing 

 complexity of the vessels that carry this fluid. The 

 heart becomes more complex, increasing from the en- 

 larged tube of the lower forms to a two-chambered 

 structure as in fishes, and finally to the four- chambered 

 heart of mammals and birds. At the same time there 

 is an increasing development of the vessels themselves, 

 and also a marked increase in their differentiation, some 

 being set aside for carrying blood from the heart to the 

 various parts of the soma, others for taking blood from 

 these parts directly or indirectly back to the heart. 



Vital activity being largely, if not wholly, a chemical 

 process, it must be accompanied by the formation of 

 waste products due to the activity of the living substance. 

 These products are usually of no more use to the animal, 

 and so must be got rid of. In the simpler forms they 

 are thrown out to the exterior, and so the process of 

 excretion, or getting rid of waste, is started. In a large 

 number of the lower forms there are no special organs 

 for this work, and it is in the worms that we find the 

 first traces of such excretory organs. There waste may 



