UNIT TWO 



Under What Conditions Can We Live? 



1 Why are there more plants and animals in some places than in others? 



2 Why are there living things in some places but not in others? 



3 Are there parts of the earth where there are no living things at all? 



4 Are there any conditions in which man cannot live? 



5 What limits the spread of mankind over the earth? 



6 How do plants and animals remain alive while inactive during the winter? 



7 Why are seeds killed if they are allowed to become damp? 



8 Why do not fish drown in water? Why can they not live in air? 



9 Why can we live longer without food or water than without air? 



Man has spread over more of the earth's surface than any other of all the 

 miUion or so species hving today. He has taken with him in his wanderings 

 some of his domesticated plants and animals, and also the fleas and worms 

 and bacteria that live on or in his body. Man has made himself at home 

 where the tiger or the bison had been master. In every region he has turned 

 to his use the native plants and animals. And he has destroyed many species 

 that he could not use, or that interfered with his plans. He wipes out a forest 

 to make room for homes and gardens and field crops. Or he pushes snakes 

 and wildcats aside to make room for cattle and chickens and dogs. 



Man is not, of course, the only wanderer. Living forms everywhere push 

 out into the surrounding regions. At the edge of a garden are weeds, and 

 beyond the weeds are cultivated plants "escaped" from the garden. After a 

 piece of land has been cleared, seedlings from the surrounding woods appear. 

 The range of every animal species changes in the same way. Most of the flies 

 that trouble us, and the vermin too, breed, of course, on the neighbors' prem- 

 ises. The locust swarms over the land, seeking what he may devour. 



Life is always on the move. But in any given situation, or with any given 

 species, life moves so far, but then meets many kinds of obstacles. The edge 

 of the ocean stops the spread of life in both directions. The very conditions 

 that enable some species to live make life quite impossible for others. 



Fishes live only in water ; the trap-door spider and the horned toad only in 

 arid regions. Butterflies flit in the air and sunshine, but tapeworms dwell in 

 the dark recesses of a little boy's intestines. The green-slime thrives on the 

 bark of a tree, but the malaria plasmodium must get inside a blood-cell. 

 Lichens live under the snows of Iceland, but Florida winters are too severe 

 for the banana. Life is truly wonderful, since it gets along under all these 

 different conditions. Y^/ no single kind of plant or animal can live under 

 all these di^erent conditions. What conditions are really essential to life? 



79 



