Life and Its Conditions. 13 
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 
