4 INTRODUCTION. 



in the proportion whicli tliese bear to one anotlier in the organism that animals 

 differ in any way Irom plr»Lls. The most characteristic of all vegetable com- 

 pounds is the one known as cellulose, very nearly allied in its chemical com- 

 position to ordinary starch. As a general rule, it may be stated that the 

 presence of an external envelope of cellulose in any organism raises a strong 

 presumption as to its vegetable nature. Still cellulose is not exclusively con- 

 fined to plants, as was at one time believed. It is now well known that the 

 outer covering of the so-called Sea-squirts or Ascidian Molluscs contains a large 

 quantity of cellulose (as much as 60 per cent in some cases). Another highly 

 characteristic vegetable product is chlorophyll, the green colouring-matter of 

 plants. Any organism which exhibits chlorophyll in any quantity as a proper 

 element of its tissues is most probably vegetable. In this case also, however, 

 the presence of chlorophyll cannot be regarded as a certain test, since it occurs 

 regularly in some undoubted animals {e.g., Stentor amongst the Infusoria, and 

 the Hydra viridis, or green Fresh-water Polype, amongst the Ccelenterata). 



Fourthly, As regards locomotive poicer^ or the ability to effect changes of 

 place at will, the results of observation are singularly at variance with our 

 preconceived notions. Before the invention of the microscope, no instances of 

 independent voluntary movements were known in plants, if we except the 

 voluntary opening and closure of flowers and their turning towards the sun, 

 the drooping of the leaves of sensitive plants under irritation, and some other 

 phenomena of a like nature. Now, however, we know of many plants which 

 are endowed, either when young or throughout life, with the power of effecting 

 voluntary movements apparently as spontaneous and independent as those ex- 

 hibited by tlie lower animals. In some cases the movements are brought about 

 by means of little vibrating hairs or cilia, with which a part or the whole of the 

 surface is furnished. When it is added that many animals are permanently- 

 fixed and rooted to solid objects in their fully-grown condition, it will be seen 

 that no absolute distinction can be drawn between animals and plants merely 

 on the ground of the presence or absence of independent locomotive power. 



Fifthly, We have shortly to consider one of the most reliable of all the 

 tests by which an animal may be separated from a plant — namely, the 

 nature of the food, and the products which are formed out of the food within 

 the body. 



The differences between animals and plants in this respect may be roughly 

 stated as follows : — 



1. Plants live upon purely inorganic substances, such as water, carbonic 

 acid, and ammonia — and they have the power of making, out of these, tnie 

 organic substances, such as starch, cellulose, sugar, kc. Plants, therefore, 

 take as food very simple bodies, and manufacture them into much more com- 

 plex substances, so that plants are the great producers in natiire. 



2. All plants which contain green colouring-matter (chlorophyll) in the pro- 

 cess of digestion break up carbonic acid into the two elements of which it is 

 composed— namely, carbon and oxygen, keeping the carbon and setting free 

 the oxygen, kn carbonic acid occurs always in the air in small quantities, the 

 result of this is that plants remove carbonic acid from the atmosphere and give 

 out oxygen. 



3. Animals, on the other hand, have no power of living on inorganic matters, 

 such as water, carbonic acid, and ammonia. They have no power of converting 

 these into the complex organic substances of which their bodies are composed. 

 On the contrary, animals require to be supplied with ready-made organic com- 



