Chap. 1 RELATIONSHIPS OF THE LIVING WORLD 3 



worms which in turn eat heavily of leaves and of soil rich in microscopic 

 plant cells. 



The watery homes of .animals are exciting because they are relatively 

 primeval. Wade into the border waters of a lively pond and you look down 

 into a world in which animals are swimming, climbing, burrowing, eating 

 plants, eating one another, mating, laying eggs, floating, and doing nothing 

 but the basic business of living. The pond is affected by surrounding condi- 

 tions but its swarming population is primarily adjusted to an ancient world of 

 water. The tidal and surface waters of the sea contain populations which 

 dwarf those of fresh water, but ponds and seas bestow similar benefits in the 

 same great boon of water. Living substances must be wet. Life began in the 

 water and all plants and animals are still bound by their need of water, even 

 though many of them have moved into deserts. All plants and animals are sub- 

 ject to the chemical and physical features of their environments. The carbon 

 cycle begins when plants take carbon dioxide from the air and build it into 

 carbohydrate food. The atmospheric pressure in high and low places and the 

 amount of oxygen in the air or water continually affect animals. 



Ecological relationships — the fish to the sea, the bird to the air — pervade 

 the evolution of animals and plants. They are apparent in a survey of the main 

 groups of animals arranged with respect to their structures and activities. They 

 also appear in special studies of certain animals as representative types, 

 such as those of the ameba, hydra, grasshopper, honeybee, and frog. In this 

 book each of these has been included with its own group of relatives instead 

 of being considered as an isolated creature; no plant or animal lives unto itself 

 alone. In the systems of the body and in their fundamental patterns animal 

 groups show resemblances and relationships. The circulatory systems of all 

 vertebrates are built on a similar ground plan. Except in protozoans and the 

 simplest of multicellular animals, kidneys are tubular organs closely associated 

 with circulating blood. From earthworm to man the body is a tube within a 

 tube; in invertebrates the nerve cord is on the ventral side of the body; in all 

 vertebrates it is on the dorsal side. Environment has been a sculptor. In envi- 

 ronment and outward form a whale is fishlike; in internal anatomy it is closer 

 to a squirrel. 



Conservation is applied ecology. Not until good things are going or gone do 

 we appreciate what they used to be. A stream runs clean and cold and well- 

 fed trout cut through its currents. This home is right for them. No alterations 

 are needed. Presently an upstream paint shop is established, the waste warms 

 and poisons the water, supplies it with scum, bad smell, and gases that kill the 

 trout. The need of getting back the clean, cold water is urgent for whatever 

 fishes may still be alive. If the paint shop and the bad smell had not become a 

 part of their environment, there would be many more alive. Conservation of 

 our natural resources is growing daily more important. The kinships of ani- 



