VON baer's principle. 101 



If we study the development of any animal, we 

 find a similar series of changes, except that tlie devel- 

 opment does not always stop with the batrachian. 

 The rabbit, for example, begins as a single animal 

 cell, an egg. It next shows the characteristics of an 

 invertebrate, a worm. Then it comes to have a two- 

 chambered heart and rudimentary gills, like a fish. 

 Next the heart becomes three-chambered and lungs 

 are developed, as in a frog or a turtle. There is so 

 much resemblance between reptiles and batrachians 

 that the two classes of animals are sometimes placed 

 in one group. The reptile is the best type of this 

 group, which would include batrachians, reptiles, and 

 birds. We may say, then, that the rabbit, at one time 

 in its history, is a reptile. But it does not stop here. 

 The heart becomes four-chambered, its body becomes 

 covered with hair, it acquires teeth, and comes to have 

 the characteristics of a mammal. 



Let us put these facts into a table : — 

 Table of Stages in the Development of a Mammal. 



This is called the Ontogenetic or Embryologic 

 Series. 



If we take all the animals that now exist and 

 arrange them in the order of their complexity, placing 

 the simplest first, we shall have another series. 



The simplest animals are one-celled animals. They 

 consist of a single animal cell, thus corresponding in 

 complexity to an egg. 



The next degree of complexity is represented 

 by the invertebrates. This is a very large group 



