INTKODUCTION. 3 



the kindred sciences of Zoology and Botany. It might have been 

 thought that nothing could be easier than to determine the animal 

 or vegetable nature of any given organism ; and such, indeed, was 

 the almost universal belief of older observers. In point of fact, how- 

 ever, no hard-and-fast line can be drawn, in the present state of our 

 knowledge, between the animal and vegetable kingdoms, and it is 

 often a matter of extreme difficulty, or even wholly impossible, to 

 decide positively whether we are dealing with an animal or a plant. 

 In the case of the higher members of the two kingdoms there is no 

 difficulty in arriving at a decision. The higher animals are readily, 

 separated from the higher plants by the possession of a distinctj^ 

 nervous system, of locomotive power which can be voluntarily exer- 

 cised, and of an internal cavity fitted for the reception and digestion 

 of solid food. The higher plants, on the other hand, possess no nerv- 

 ous system or organs of sense, are incapable of voluntary changes of 

 place, and are not provided with any definite internal cavity, their 

 food being wholly fluid or gaseous. 



The lower animals [Protozoa) cannot, however, be separated in many cases 

 from the lower plants {Protophyta) liy these distinctions, since many of the 

 former have no digestive cavity, and are destitute of a nervous system, and 

 many of the latter possess the power of active locomotion. In determining, 

 therefore, the nature of these amhiguous organisms, tlie following are the chief 

 points to he attended to : — 



Firstly, As to mere/onn. or external configuration, no certain rules can he 

 laid down for separating animals and plants. Many of tlie lower plants, either 

 in their earlier stages of existence or when grown up, are exactly similar in 

 form to some of the lower animals. This is the case, for example, in some of 

 the Algce, which closely resemble some of the Inlusorian animalcules. Many 

 ■undoubted animals, again, are rooted to solid objects in their adult state, and 

 are so plant-like in appearance as to be always popularly regarded as vegetables. 

 This is the case with many of the so-called Hydroid Zoophytes, such as the Sea- 

 firs, and also with the much more highly organised Sea-mats {F/iistra), all of 

 wliich are usually regarded as sea-weeds by seaside visitors. This is also, but 

 less strikingly, the case with the Corals and Sea-anemones, of which the latter 

 are often spoken of as " sea-flowers, " 



Secondly, No decided distinction can be drawn between animals and plants 

 as to their minute internal structure. Both alike consist essentially of minute 

 solid particles (molecules or granules^ of cells, or of fibres. 



Thirdly, As regards chemicaZ composition, there are some decided, though 

 not universal, differences between plants and animals. As a general nde, it 

 may be stated that plants exhibit a decided predominance of what are known 

 to chemists as " ternary compounds " — that is to say, compounds which, like 

 sugar, starch, and cellulose, are composed of the three elements, carbon, hydro- 

 gen, and oxygen. They are, comparatively speaking, poorly supplied with 

 "quaternary" compounds, which contain the fourth element, nitrogen, in 

 addition to the three first mentioned. Animals, on the other hand, are rich in 

 quaternary nitrogenised compounds, such as albumen or fibrin. Still, in both 

 kingdoms we find nitrogenised and non-nitrogenised compounds, and it is only 



