THE VERTEBRATES AND THEIR ENVIRONMENT 35 



In examining such a chain of events, one should remember that 

 it is no stronger than its weakest link. If the connection breaks 

 at any one place, the conclusion does not follow from the pre- 

 mises. It is quite possible that the foregoing relationship between 

 cats and clover may not obtain, because one or more links of the 

 sequence do not actually occur. Probably in no such case can we 

 ever be sure that the relationships we observe are in reality matters 

 of life and death in a large number of cases. Nevertheless, in view 

 of the many glimpses of such relationships that are everywhere 

 disclosed by organic nature, we can be sure that living beings are 

 frequently inter-related in ways that are of the utmost impor- 

 tance to the organisms concerned ; and that such interdependence 

 of life upon life is one of the major factors in the complex of ani- 

 mate nature. 



As explained under the head of definitions in the preceding chap- 

 ter, the biological science that deals with the relationship of organ- 

 isms to their environment, both living and non-living, is known as 

 Ecology. The ecological study of vertebrate animals presents 

 many problems that are intimately related to problems of human 

 existence. Thus, man, although no longer prej^ed upon by the 

 larger animals, falls a victun to the attacks of parasites in the form 

 of the germs of disease, or is tortured by insect pests which secure 

 food from his body. Insects prey upon the plants that he cul- 

 tivates for food. Insect-eating birds in turn devour the insects 

 and are themselves destroyed by their enemies. From a state in 

 which he lived by dint of holding his own physically with other 

 forms of life, man has come to live by his wits, and in this way has 

 become the dominant species among those now in existence. 

 Within a very brief space of time, this single species. Homo sapiens, 

 has upset the balance of power which had previously existed in 

 the inter-relationships of Hving things throughout the earth and 

 had continued throughout geologic time. A new and terrible 

 thing has come upon the world of Hving beings. Because of man, 

 forests have given place to fertile fields or to bare and eroded hill- 

 sides, the larger forms of animal Hfe that do not serve for domesti- 

 cation are fast becoming extinct, and introduced animals and 

 plants are supplanting native stocks in many localities. The 

 zoologist can but feel that the world will be a far less satisfac- 

 tory place for man, to say nothing of the animals, if the birds 

 become mostly English sparrows and the mammals rats and mice, 



