PART III 



PRINCIPLES OF BIOLOGY ILLUSTRATED BY 

 TYPES OF ANIMALS 



WE have seen in Part II how types of plants illustrate the 

 principles of biology which were introduced by study of the 

 bean plant, and also give us a wider acquaintance with rep- 

 resentatives of the various groups of plants which, while 

 widely different in form, are remarkably similar in their 

 fundamental life-activities. We shall now make a series 

 of parallel studies on the animal side of, biology. In study- 

 ing plants we began with the bean, which belongs to the 

 highest plants ; and then in succession we passed down the 

 scale of plant life to conifers, ferns, mosses, algae, fungi, and 

 the simplest plants. In our study of types of animals it will 

 be interesting to reverse this order, and instead of going 

 gradually down to the simplest animals, we shall pass directly 

 from the frog to the one-celled animals, and afterwards 

 examine a number of types of animals representing the gradu- 

 ally increasing complexity of animal structure up to the 

 highest vertebrates. 



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