Chap. I.] INTRODUCTORY. 



Animals cannot do this. They require ready-made protoplasm 

 in the form of vegetable or animal matter. Plants raise the 

 inorganic into the organic; animals then take up the process 

 and often carry it forward to more complex products. But no 

 animal can raise the inorganic into the sphere of the organic. 

 That is the function of the plant. Plants alone can manufacture 

 protoplasm. Animals require the manufactured article. 



But the manufactured article is not directly incorporated as 

 such into the substance of the consumer. The protoplasm of 

 sheep cannot become the protoplasm of man without being 

 largely taken to pieces, chemically, and then put together again. 

 Hence the necessity for digestive and assimilative organs to 

 enable the animal to do this. Even the protoplasm of the 

 mother's milk must be digested before it can be assimilated by 

 the young. In plants anything of the kind is exceptional and 

 subsidiary. They take the elements of protoplasm from the air 

 that bathes their leaves, and from the water that bathes their 

 roots. Hence the branching and spreading form of that part 

 which is exposed to the air, and the far-reaching ramifications 

 of that part which is implanted in the earth. We may sum up 

 this distinction by saying that animals differ from plants in that 

 they require protoplasmic food-stuff which must undergo a more 

 or less- complex process of digestion, within or without their 

 bodies, before it can be assimilated. 



Out of this main distinction there flows a secondary distinc- 

 tion of some importance. Plants manufacture organic tissues 

 out of such inorganic raw materials as contain the requisite 

 elements. One of these raw materials is the carbonic acid gas 

 of the air, from which the green plant abstracts the carbon, 

 returning the oxygen to the air as a by-product for which it has 

 no use. But carbonic acid gas is one of the chief products of that 

 continued waste which is entailed by the ceaseless activity of the 

 animal. And to enable that waste which, as we have seen, is 

 essential to animal life to continue, the animal requires a more 

 or less liberal supply of oxygen. So that animals and the 

 higher green plants perform opposite and complementary func- 

 tions in the economy of nature. By the plant carbonic acid gas 



