PART III 



THE FORMS OF ANIMAL LIFE 

 CHAPTER XI 



THE ELEMENTS OF STRUCTURE 



s . The Resemblances and Contrasts between Plants and Animals — 

 2. The Relation of the simplest Animals to those which are 

 more cofnplex — 3. The Parts of the Animal Body: Organs, 

 Tissues, Cells 



The study of form and structure (Morphology), and the 

 study of habit and function (Physiology), are both as essen- 

 tial to science as the realities are in life. It is with the 

 forms of animal life and their structure that we have now 

 to do, but it seems useful at the outset to compare plants 

 and animals. 



I. The Resemblances and Contrasts between Plants 

 and Animals. — Every one could point out some differences 

 between a tree and a horse, but many might be puzzled to 

 distinguish clearly between a sponge and a mushroom, 

 while all have to confess their inability to draw a firm line 

 between the simplest plants and the simplest animals. For 

 the tree of life is double like the letter V, with divergent 

 branches, the ends of which, represented, let us say, by 

 a daisy and a bird, are far apart, while the bases gradually 

 approach and unite in a common root. 



Plants and animals are alike, though not equally, alive. 



