282 LIVES OF PLANTS AND ANIMALS 



above the timber line. We might think that these different regions 

 (life zones) were due entirely to temperature. On the mountain- 

 side this is largely true, but we find that all the factors we have 

 already discussed are at work determining the places where living 

 things shall be found. The amount of water present in the soil, 

 the kinds of soils there, the ranges in the temperature, the wind, 

 and many other factors play their parts. One big problem in the 

 geographical distribution of organisms is to find, first, adaptations 

 of organisms for life in various localities, and then the place 

 where a certain kind of living thing has made its start in an area, 

 and finally how life spreads from this area to other areas. 



On the whole, nature establishes a balance in life and there 

 is give and take between all living things. That part of biology 

 which deals with the relationships of organisms to each other and 

 to their environments is called ecology. 



PROBLEM I. WHAT GENERAL BIOLOGICAL RELATIONS 

 EXIST BETWEEN PLANTS AND ANIMALS? 



Each place where plants and animals are found living togethei 

 supports a characteristic community of plants and animals. This 

 living together seems to be determined largely by the conditions 

 of the environment in which they are placed. Groups living 

 together in a pond or slow-flowing stream will be quite different 

 from groups living in a rapid mountain stream or in the ocean. The 

 communities of animals and plants living in dry or desert localities 

 would differ to a still greater extent from any of those first men- 

 tioned. Only certain animals and plants will five and flourish in an 

 indoor aquarium, but in all such groups the animals and plants 

 living together have certain fundamental relationships to each other 

 and to their surroundings. 



Study of a balanced aquarium. Perhaps the best way for us 

 to understand this relation between plants and animals is to 

 study an aquarium in which plants and animals five and in 

 which a balance has been established between the plant fife on 

 one side and animal life on the other. Here we see many evidences 

 of the relationship between the environment and the living things 

 in the water. The plants are buoyed up by the water, they do not 



