6 INTRODUCTION. 



on irritation ; the Dionsea muscipula, the Venus' Fly-Trap, the extremities 

 of whose leaves have the power of closing on insects or other bodies 

 brought into contact with them. Plants are also possessed of internal 

 motion: witness the circulation of the sap and the circulatory motions 

 in the interior of many vegetable cells. They also turn spontaneously 

 to the light and extend their rootlets to the most nutritive soil. 



Again, all animals are not possessed of the power of motion. 

 Sponges, coral polyps, hydroid zoophytes, sea-mats, etc., are* entirely 

 destitute of locomotive power, and spend their entire existence rooted 

 fast to some immovable object. Hence, the possession of motor power 

 is not characteristic of animal life, and its absence does not prove the 

 organism to be a vegetable. 



Chemical anal3'sis helps us but little more in the attempt to dis- 

 tinguish animals frojn vegetables. Carbon and nitrogen compounds form 

 a large proportion of the constituents of each, and a large number of 

 complex combinations found in animal tissues are represented by entirely 

 similar compounds in vegetable matter. There is therefore no one chemi- 

 cal compound whose presence is characteristic of animality or vegetable 

 nature ; for " cellulose," the substance out of which wood-fibre and the 

 walls of plant-cells are formed, has been ascertained' to form the greater 

 part of the external coverings of certain molluscous animals (ascidians). 

 So also chlorophyll, the green coloring matter of plants, is the cause of 

 the green color of many infusorial animalcules and of Hydra viridis, 

 while starch has been found in the ventricles of the brain of animals, 

 and is represented by glycogen, a body closely alialogons to starch and 

 manufactured by the animal economy. Such examples, therefore, show 

 that chemical examination can give us no definite aid in separating plants 

 and animals. 



The microscope is also powerless to give us an infallible rule which 

 will enable us to distinguish animal from vegetable tissue. In other 

 words, plants and animals are built up on the same general plan ; their 

 intimate structure closely coincides. Both originate in cells, consisting, 

 in their typical form, of a cell-wall, cell-contents, or protoplasm, — nucleus 

 and nucleolus, — and in both the parent cell undergoes subdivision and 

 results in the birth, growth, and development of myriads of other cells, 

 constituting the tissue of the plant or animal, and differing no more from 

 each other than almost any mature animal or vegetable cell does from 

 the germ from which it originated. 



Nor is the possession of a digestive cavity, mouth, or alimentary 

 tube characteristic of animals ; for there are vegetables which possess a 

 stomach, as the Nepenthes, or Pitcher-Plant, which has a cavity cor- 

 responding to a stomach, in which digestive fluids are poured out, and 

 in which digestion and absorption take place. On the other hand, many 



