that is because they live in partnership with bacteria that are able to combine, 

 or "fix", nitrogen from the air into compounds that the larger partners can 

 use (see page 152). 



Jack Sprat Principle You recall that Jack Sprat could eat no fat, 

 whereas his wife could eat no lean. These two people did not let differences 

 in taste cause ill-feeling and bickering. Instead, according to legend, they 

 managed amicably and sensibly to make the most of their undoubtedly 

 limited resources. They licked the platter clean, and we may assume that 

 both continued to be well nourished. At any rate, we can observe this prin- 

 ciple of specialization at work when we consider the wide variety of condi- 

 tions under which different species of plants and animals thrive. The most 

 obvious specialization is, of course, between water-dwelling species and land- 

 dwelling species. There are many species of plants and animals, however, that 

 live on the margin between land and water — marsh plants and animals, tide- 

 water forms, and so on. Thus ferns, mosses, skunk cabbages, and certain fungi 

 thrive along woodland streams, but are seldom found growing in open fields. 

 Muskrats, cattails, sedges and red-winged blackbirds are associated in marshes 

 or swamps. The amphibians are typical in-between forms, the very life cycle 

 of the frog being adapted to alternation of wetness and dryness. However, 

 Hving in air and Hving in water involve such great differences in structure 

 and in behavior that most species live in either one medium or the other. 



Adaptation to Change The emergence of many species may be looked 

 upon as an adaptation — in the course of time — to new situations into which 

 living beings are forced by the pressure of population. We have seen that 

 many specialized types of plants and animals do in effect fill in gaps among 

 other species. We may see this more clearly if we consider what happens when 

 a decisive change takes place in climate, for example, or in a river when 

 industrial wastes are discharged into it. 



Let us imagine a relatively dry region occupied by plants of many species 

 and a corresponding population of animals. The speciaUzed types fit the physi- 

 cal surroundings — the soil and its chemical contents, the moisture, the tem- 

 perature, the sunshine. And they fit one another — taller plants and low 

 growths, the insects and the worms, the bacteria and the birds, all make up a 

 fairly constant mixture season after season. But now, if this region should 

 become flooded, a large proportion of both plant and animal inhabitants would 

 be destroyed. Only those that were not too highly speciaUzed would survive, 

 mostly simple plants and animals that can endure a great range of dryness or 

 moisture. Those that are too finicky or else too rigid would be killed off. A 

 marked change in physical conditions always destroys some species. 



On the other hand, as the water destroys thousands of individuals of many 

 species, it also favors certain other species^ — less specialized water-dwellers or 

 forms that thrive in wet situations. Life is destroyed; but life goes on. 



550 



