38 THE STEUCTUEAL UNITS OP UVING JIATTER 



Evolution. — The theory has been set forth in what is 

 known as the doctrine of evolution that all plants and 

 animals have arisen from a common origin, namely, the 

 substance protoplasm, and that each form of plant or 

 animal has developed from a less complex form. According 

 to this theory living matter once consisted of simple 

 one-celled forms, and all the complicated forms we now 

 know have been developed from these, much as the many 

 kinds of houses in a city have been made by different 

 arrangements of the bricks. 



Classification. — In the animal world it is customary to 

 group the various forms in an ascending series with the 

 most complex form (man) at the head of the series. When 

 this is done it is found that the series is roughly divisible 

 into two groups, vertebrates, or animals with a back- 

 bone; and invertebrates, or those without a backbone. 

 In the former class are included all the fishes, frogs, 

 reptiles, birds, and mammals. In the latter are the one- 

 celled protozoans, the worms, mollusks, insects, etc. 

 Man, in common with horses, cows, rabbits, dogs, cats, 

 etc., possesses hair, and glands for suckling its young 

 (mammary glands), and hence belongs to the group of 

 mammals. It is for this reason, namely, that all mam- 

 mals ar« very much alike in structure, that it is possible 

 for us to use the dissections of other mammals, such as 

 the cat, rabbit, and the rat, to get a clear idea of our 

 own structure. Many times in the pages to come we 

 shall make use of this similarity of structure for illustra- 

 tions of structural arrangement. 



