ECOLOGICAL 201 



plants themselves, and of all the animals that feed on plants. Even 

 when the animal is a thoroughgoing carnivore, a few links in the 

 nutritive chain bring us back to green plants. All flesh is eventually 

 grass and all fish is eventually diatom. The green plant, whether 

 grass or diatom, finds the raw materials of its food in carbon dioxide, 

 water, and dissolved salts ; and the synthetised nutritive compounds 

 — carbohydrates, fats, and proteins — are abundant enough to sus- 

 tain the animal world as well as the plants themselves. Locally and 

 temporarily, as in plagues of voles or of locusts, the animals may 

 devour all the available plants; but it is plain that this is a rare, 

 not a normal, occurrence. On the whole, the nutritive balance is 

 preserved. 



Not less important, though less frequently realised, is the fact 

 that green plants have made the oxygen of the air, on which animal 

 life depends. The original atmosphere of the earth was rich in 

 carbon dioxide and water vapour; it had relatively little nitrogen 

 and very little oxygen — the production of which has been and con- 

 tinues to be to the credit of green plants. 



The Living and the Dead.— Surprise has often been ex- 

 pressed at the fact that we do not usually see many dead animals 

 lying about. After storms the flat beach is sometimes strewn with 

 sponges, zoophytes, jelly-fishes, starfishes, sea-urchins, and molluscs, 

 which are thrown up in profusion by some peculiar combination of 

 wind and wave, but on land it is very rarely that we see any analogue 

 of this jetsam. We have known of two hundred small birds being 

 gathered in one farm-yard after a night of very severe frost, but 

 such an occurrence is so rare in North Temperate countries that we 

 remember it all our life. Part of the reason for the rarity of dead 

 animals is that so many creatures are devoured by others; and 

 another part of the reason is that there are numerous scavenger 

 animals, such as sexton beetles and the larvae of carrion flies, which 

 bury or do away with the dead bird or mammal. Deeper, however, 

 is the role of Bacteria, which are of great assistance in securing the 

 smooth working of Nature. A dead animal rots; that is to say, its 

 tissues are broken down by Bacteria and converted in course of 

 time into salts, ammonia, and water. What is restored to the soil 

 may soon be absorbed by the roots of plants, and even the ammonia 

 that steals off into the air may be recaptured and brought again 

 into the service of life. Thus Bacteria complete a wide circle; they 

 unite dead animal and living plant. 



Nutritive Chains.— There are many familiar illustrations of 

 what may be called a nutritive balance between different kinds 

 of animals. Thus gulls often eat fishes, and fishes often eat crusta- 

 ceans, and crustaceans often depend on diatoms; and some sort of 

 balance must be sustained, year in and year out. A correlation has 

 been convincingly worked out between the catch of mackerel, the 



