5 



Animals and Tlieir Environments 



Animals abound in great numbers. Thrust a stick into a large ant nest on a 

 July day and millions of ants pour out, many carrying white packages that 

 taken altogether contain myriads of their eggs and young ones. Sea birds 

 scarcely have room to sit on their eggs during the great gatherings of the breed- 

 ing season (Fig. 5.1). Populations of animals, except the human ones, seem 

 to stay about the same size, but those that have been carefully observed have 

 proved quite the opposite. The dips and peaks in the populations of one kind 

 of animal also affect others. In Labrador in a recent year the numbers of 

 field mice ran up to a peak and the hawks and snowy owls grew fat; in another 

 year they almost vanished becoming so scarce that the snowy owls flew down 

 to New England for better eating. 



Animals enter every part of the earth except craters of active volcanoes and 

 places poisoned by civilization. They abound in the damp tropics. Microscopic 

 organisms crowd the surface waters of arctic seas, for cold water holds more 

 oxygen than warm water and food is abundant. On their journey into the 

 Antarctic members of the Robert Scott Expedition found emperor penguins 

 incubating their eggs, holding them on the tops of their feet in the dark of the 

 antarctic winter "with the temperature seventy degrees below frost and the 

 blizzards blowing." 



The Numbers of Species. The term species is commonly used but difficult 

 to define. Animals of one species resemble one another, interbreed with one 

 another and do not usually interbreed with animals of other such groups. The 

 number of described species is still growing. For birds and mammals it may 

 for the present be nearly complete; for protozoans and insects it is far from 

 that. Frequent estimates suggest that only ten per cent of all insects is yet 

 accurately described. In 1946 the total number of known living species of ani- 

 mals was figured at about one million (Fig. 5.2), 



Variety and Similarity. Large numbers of animals have basic similarities; 

 they also have many less fundamental differences. Likenesses and differences 



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