24 



COMPARATIVE ZOOLOGY. 



a similar simple cellular structure ; and all animal tissues, 

 while forming, are cellular. But this character, which is 

 permanent in plants, is generally transitory in animals. 

 In the more highly organized tissues the cells are so united 

 as partly or wholly to lose their individuality, and the 

 characteristic part of the tissue is the intercellular sub- 

 stance, while the cells themselves are small and unimpor- 

 tant, or else the cells are melted together and lose their 

 dividing walls, as in striped muscles and in nerves. Ex- 

 cepting the lowest forms, animals are more composite than 

 plants, i. e., their organs are more complex and numerous, 

 and more specially devoted to particular purposes. Kep- 

 etition of similar parts is a characteristic of plants ; and 

 when found in animals, as the Angle-worm, is called vege- 

 tative repetition. Differentiation and specialization are 

 characteristic of animals. Most animals, moreover, have 

 fore-and-aft polarity ; in contrast, plants are up-and-down 

 structures, though in this respect they are imitated by 

 radiated animals, like the Star-fish. Plants are continually 

 receiving additional members ; most animals soon become 

 perfect. 



(5) Physiology. — In their modes of nutrition, plants and 

 animals stand widest apart. A plant in the seed and an 

 animal in the egg exist in similar conditions: in both 

 cases a mass of organic matter accompanies the germ. 

 When this supply of food is exhausted, both seek nourish- 

 ment from without. But here analogy ends : the plant 

 feeds on mineral matter, the animal on organic. Plants 

 have the power to form chlorophyl, the green coloring 

 matter of leaves, which uses the force of the sunlight to 

 form starch out of the inorganic substances — carbon-di- 

 oxide and water. They are able also to form albuminoid 

 matter out of inorganic substances. A very few animals 

 which have a substance identical with or allied to chloro- 

 phyl have the same power, 7 but in general animals are de- 



