EVOLUTION 



193 



Increasing Complexity of Structure and of Habits in Plants and 

 Animals. In our study of biology so far we have attempted to 

 get some notion of the various factors which act upon living things. 

 We have seen how plants and animals interact upon each other. 

 We have learned something about the various physiological pro 

 cesses of plants and animals, and have found them to be in many 

 respects identical. We have found grades of complexity in plants 

 from the one-celled plant, bacterium or pleurococcus, to the com 

 plicated flowering plants of considerable size and with many 



formations inWestern United States and Characteristic Type cf 'Horse in each 



The geological history of the horse. (After Mathews, in the American Museum 

 of Natural History.) Ask your teacher to explain this diagram. 



organs. So in animal life, from the Protozoa upward, there is 

 constant change, and the change is toward greater complexity of 

 structure and functions. An insect is a higher type of life than a 

 protozoan, because its structure is more complex and it can per 

 form its work with more ease and accuracy. A fish is a higher 

 type of animal than the insect for these same reasons, and also for 

 another. The fish has an internal skeleton which forms a pointed 

 column of bones on the dorsal side (the back) of the animal. It is 

 a vertebrate animal. 



HUNTER, CIV. BI. 13 



