Life and Its Conditions. \ 3 



constituents of carbon and oxygen, retaining the former and 

 setting the latter free. And as the atmosphere always con- 

 tains carbonic acid in small quantities, the result is that 

 plants remove carbonic acid therefrom and give out oxygen. 

 Animals, on the other hand, have no power of living on 

 water, carbonic acid and ammonia, nor of converting these 

 into the complex organic substances of their bodies. That 

 their existence may be maintained animals require to be 

 supplied with ready-made organic compounds, and for these 

 they are all dependent upon plants, either directly or indi- 

 rectly. In requiring as food complex organic bodies, which 

 they ultimately reduce to very simply inorganic ones, 

 animals are thus found to differ from plants. Whilst plants 

 are the great manufacturers in nature, animals are the great 

 consumers. Another distinction, arising from the nature of 

 their food, is that animals absorb oxygen and throw out 

 carbonic acid, their reaction upon the atmosphere being 

 exactly the reverse of that of plants. There are organisms, 

 it must be understood, which are genuine plants so far as 

 their nutritive processes are concerned, but which, neverthe- 

 less, are in the possession of characters which could locate 

 them among the animals. Volvox, so abundant in our 

 streams during the proper seasons, affords a splendid illus- 

 tration of the truth of this statement. Plants, which are 

 devoid of chlorophyll, as is the case with the Fungi, do not 

 possess the power of decomposing carbonic acid under the 

 influence of sunlight, but are like animals in requiring 

 organic compounds for their food. Two points must there- 

 fore be borne in mind in regarding the general distinctions 

 between plants and animals which we have thus briefly out- 

 lined, and these are that they cannot often be applied in 

 practice to ambiguous microscopic organisms, and certainly 

 not to plant-forms that are destitute of chlorophyll. 



That life should manifest itself certain conditions are essen- . 

 tial, but some of which, though generally present, are not 

 absolutely indispensable. One condition, however, seems to 



