INTEODUCTION. 5 



pounds if their existence is to be maintained. These tliey can only get in the 

 iirat place from plants, and therefore animals are all dependent npon plants 

 for food either directly or indirectly. Animals, therefore, dift'er from plants 

 in requiring as food complex organic bodies which they ultimately reduce to 

 very much simpler inorganic bodies. Whilst plants, then, are the great manu- 

 facturers in nature, animals are the gi'eat consumers. Another distinction 

 arising from the nature of their food is, that whilst plants decompose carbonic 

 acid, keeping the carbon and setting flee the o.xygen, animals absorb oxygen 

 and give out carbonic acid, so that their reaction upon the atmosphere is the 

 reverse of that of plants. It should be added, however, tliat there are organ- 

 isms which are genuine j^hants so far as concerns their nutritive jtrocesses, but 

 which, nevertheless, exhibit other characters which would place tliem among 

 animals. Examples of such organisms are found in Volvox and its allies. 



As regards these general distinctions between plants and animals, there are 

 two points which should be remembered : — 



1. That even if universally true, these distinctions can often not be applied 

 in practice to tlte ambiguous microscopic organisms about M'hich alone any 

 doubt can be entertained. 



2. These general laws are certainly not of universal application in the case 

 of plants. Thus, plants which do not contain chlorophyll {e.(/. Fungi) do not 

 possess the power of decomposing carbonic acid under the inHuence of sun- 

 light, but are like animals in requiring organic compounds for their food. 



3. Conditions of Life. 



Certain conditions are necessary for the manifestation of life or 

 vitality, as, indeed, is the case witli every known force. Some of 

 these conditions, though very generally present, are not absolutely 

 indispensable ; but there is one condition which appears to be 

 essential, and that is, that the living body nnist be composed of a 

 certain material. This material forms the essential and funda- 

 mental parts of every living being, whether vegetable or animal, 

 and it is usually known by the technical name of "protoplasm" 

 (Gr. protos, first ; and plasso, I mould). Living beings may, and 

 often do, contain in their bodies many other substances besides 

 " protoplasm," but it is in this substance that vitality seems alone 

 to be inherent ; and we are consequently justified in saying that 

 life is only found in connection with protoplasm. It is, however, 

 still a matter of opinion whether the protoplasm of all animals and 

 plants is precisely the same, and under any circumstances we may 

 believe that this substance is simply the medium or vehicle through 

 which vital force manifests itself. Using the term in its general 

 sense, " protoplasm " is chemically related in its nature to alhumen 

 or white-of-egg, and generally has the character of a jelly-like, 

 semi-fluid, transparent material, which does n(it in itself exhibit 

 any definite structure. When heated to a certain temperature, it 

 solidifies or " coagulates," just as the white of an egg does when 

 2 



