INTRODUCTION 



other living things, the facts which render it necessary to recognise 

 two great primary grades of animals a lower called the Protozoa 

 and a higher called the Metazoa. 



A. THE DIVIDING-LINE BETWEEN PLANTS AND ANIMALS. 



Living things Bionta are without difficulty, and by the 

 general agreement of both skilled naturalists and the observant 

 layman, divided into two greatly differing groups or series, the 

 animals or Zoa and the plants or Phyta, and into those two great 

 groups only. The study of the one series is called Zoology, and of 

 the other Phytology, or more usually Botany. It is easy to lay 

 down certain general propositions by which nearly all animals are 

 distinguished from nearly all plants. The distinctions which can 

 be thus indicated all arise from one great difference in the chemical 

 activity of the living substance of an animal as compared with that 

 of a plant. Although the living substance of both animals and 

 plants, to which Hugo von Mohl gave the name Protoplasm, appears 

 in both series in the form of nucleated corpuscles called cells, 

 and although the formal appearances and the range of chemical 

 activities exhibited both by the general protoplasm and by the 

 nuclear structures of the cells of animals and plants are practically 

 identical, yet there is a predominant difference in the habitual 

 exhibition of their activities which separates animals from plants, 

 and has determined the difference of form and activity characteristic 

 of the living things assigned to either of the two groups. 



Living protoplasm, whether of animal or plant, undergoes 

 (when the processes of life are not, as they may be for a short or 

 for a very extended period, suspended) constant chemical change, 

 requiring the access of free oxygen to the protoplasm and the 

 consequent oxydation of some of its material which becomes 

 " wasted " or lost and carried away by diffusion from the living 

 protoplasm. This loss has to be replaced, and the process by which 

 it is replaced is " nutrition " ; the material taken by a living thing 

 for the purposes of nutrition is its "food." 



The result of nutrition is not limited to the repair of loss in 

 the living thing, but is for a part or the whole of its existence 

 in excess of the loss ; so that increase of the bulk of the living 

 material or "growth" is a result. The elements carbon, hydrogen, 

 oxygen, and nitrogen, combined to form molecules of the highest 

 degree of complexity, are the essential constituents of living 

 material. It is these that are oxydised and wasted and pass from 

 the living thing during life : it is these which have to be replaced. 



Animals are unable to assimilate, that is, to utilise as food, the 

 simpler chemical compounds of carbon or of nitrogen. They can 

 only take their nitrogen from food which is in the elaborate form of 



