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. 

 \Ve 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 



Ktnnations in Western United Slates and Characteristic Type cf Horse in each 



Protchippus 



Be cent 

 Pleistocene 



d r-r-fh 



^ 



-T-^-4 I- I- =f= I- - 1 ) 



intn^ 





Meschippus 



Prctcrchippus 



ffyracotherium 

 (Eohippus) 



Fore Foot 



One Toe 

 Splints of 



o* 



a Three Toes 



Side toes 

 l-^nct touching 

 $the ground 



Tees 



Side toes 



M touching 



ft the ground 



Splint o 



FourTces 



Four- Toes 



Splint of 

 1 s -' digit 



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 



