CHAPTER I 

 THE FIELD OF ZOOLOGY 



A wealth of animal life about us challenges our attention. Birds 

 travel the highways of the air. Vegetation of all kinds swarms with 

 animals. The surface of the earth is ahve with crawling things, and 

 under many objects lying on the ground an animal community is hidden. 

 The ground itself teems with life. And not only are fresh waters abun- 

 dantly populated, but the sea, which has been supposed to mother all 

 life, is occupied by a host of forms, greater in variety than those of any 

 other environment. 



1. Appeal of Zoology. — Among this vast assemblage are animals 

 which appeal to us because of their beauty or oddity of appearance; 

 others to which we are attracted by their remarkable and interesting 

 activities; still others whose varied and complex relationships to one 

 another excite our wonder and suggest a multitude of questions; and, 

 finally, many whose relations to ourselves, either beneficial or injurious, 

 demand our serious consideration. 



2. Number of Animals. — The number of individuals which at any 

 given time is living in this world surpasses calculation and is beyond the 

 power of the imagination to conceive. That large number, subject to 

 the modifying influences of changing seasons and affected from time to 

 time by an altering of the balance between animals of different kinds, is 

 constantly maintained. Naturally great differences exist between differ- 

 ent regions of the earth's surface, which, in very different degrees, offer 

 the conditions favorable for animal life. 



3. Variety of Animals. — The number of kinds of animals is not yet 

 determined and probably never will be precisely known. Those living 

 which have been previously described and named have been estimated 

 at approximately 600,000; and there is no doubt that if all still unknown 

 were added, the total would far exceed a million. The exact number, 

 however, is subject to constant change, since some kinds of animals are 

 probably continually becoming extinct and new ones are probably as 

 continually being developed. This enumeration also takes no account 

 of the millions of species which have Uved in the past and have perished, 

 some without leaving any trace, others represented more or less com- 

 pletely by fossils. Then, too, opinions differ greatly as to what con- 

 stitutes the difference between two kinds or, in other words, what 

 constitutes a species. The words type and form are frequently used in 

 the same sense as species, or kind. 



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