DIFFERENCES BETWEEN ANIMALS AND PLANTS. 25 



an exception to this law. The reaction of plants upon the 

 atmosphere is therefore characterised by the production of 

 free oxygen. Animals, on the other hand, absorb oxygen 

 and emit carbonic acid, so that their reaction upon the 

 atmosphere is the reverse of that of plants, and is charac- 

 terised by the production of carbonic acid. 



Finally, it is worthy of notice that it is in their lower and 

 not in their higher developments that the two kingdoms of 

 organic nature approach one another. No difficulty is ex- 

 perienced in separating the higher animals from the higher 

 plants, and for these universal laws can be laid down to 

 which there is no exception. It might, not unnaturally, 

 have been thought that the lowest classes of animals would 

 exhibit most affinity to the highest plants, and that thus a 

 gradual passage between the two kingdoms would be estab- 

 lished. This is not the case, however. The lower animals 

 are not allied to the higher plants, but to the lower ; and it 

 is in the very lowest members of the vegetable kingdom, or 

 in the embryonic and immature forms of plants little higher 

 in the scale, that we find such a decided animal gift as the 

 power of independent locomotion. It is also in the less 

 highly organised and less specialised forms of plants that 

 we find the only departures from the great laws of vegetable 

 life, the deviation being in the direction of the laws of 

 animal life. 



