knowledge and skill fail to make such efforts go. There are new enemies never 

 encountered before. In their attempts to penetrate tropical regions, Euro- 

 peans have been for several centuries obstructed by the new kinds of diseases 

 and pests. Where many different plant and animal species have become es- 

 tablished, man's arrival often interferes with conditions seriously. We some- 

 times destroy what we should like to preserve, or else increase forms that we 

 find objectionable. 



We have already seen that many of our cultivated plants depend for com- 

 pleting their life cycles upon the co-operation of certain insects (see page 408). 

 Other cultivated plants are destroyed by other insects. To preserve and mul- 

 tiply our plant and animal populations, we have to look after many other 

 species — encourage some and destroy others. To ensure a human population, 

 it is not enough to establish physical and chemical conditions that favor cul- 

 tivated plants and animals. We need further to guard against bacteria, pro- 

 tozoa and worms that cause disease, and against mosquitoes, fleas, flies, various 

 rodents and other carriers of infection. 



To make the earth support more human beings, it becomes necessary to 

 control the distribution and the density of hundreds of other species — some 

 of them directly useful, of course, but others important in various indirect 

 ways. Some affect the health of humans and of our cultivated organisms. 

 Some supply food for our cattle and other domestic animals. Some affect the 

 physical conditions in ways that are important. No man can live by himself 

 alone; but it seems that no other species can live by itself alone. 



In Brief 



Life is self-limiting and in every cell there is an orderly succession of stages 

 from beginning to end. 



Among simple organisms a line of protoplasm can remain alive indefinitely, 

 whereas in the more complex, many-celled body it cannot. 



As in social organization, organic specialization makes it possible to use 

 available materials and energies more efficiently and to carry on life under 

 new conditions. 



Increased specialization involves a more complete and more delicate co- 

 ordination, which is accompanied by a lessened capacity of the parts to re- 

 generate and to adjust themselves. 



Growing and developing and reproducing arc possible for some individuals 

 only on condition that other living things are destroyed. 



A species increases in numbers as favorable conditions arise, or as it moves 

 into favorable environments; man goes further and increases in numbers as 

 he finds ways of adjusting a great variety of environments to his needs. 



537 



