330 THE INFLUENCE OF LIVIKG SDEROUNDINGS, 



SECTION III. 

 THE INFLUENCE OF LIVING SURROUNDINGS. 



CHAPTER XI. 



THE TRANSFORMING INFLUENCE OF LIVING ORGANISMS ON 

 ANIMALS. 



Introductory Remarks. — It is self-evident that all animals, 

 without exception, are to a certain degree simiiltaneously depen- 

 dent on various other animals as well as on plants ; for even 

 such species as appear to depend for their nourishment exclu- 

 sively on a particular kind of vegetable food also come under 

 the indirect influence of other organisms, often of a great num- 

 ber, by reason of that very limitation. Examples of such 

 highly complicated relations and interdependence are familiar 

 to all, and this relieves me from the necessity of repeating here 

 all that has been said so often and with so much emphasis by 

 Darwin and others on this part of my subject. 



Occasionally alterations taking place in these complicated 

 relations may even lead — as is actually known — to the destruc- 

 tion of a species. For instance, if the food-plant supplying a 

 strictly monophagous animal is by any circumstance extirpated, 

 that species must inevitably die out. When plants on which 

 any species of animal is dependent for food are affected and 

 modified by any accidental variation that may occur in the 

 temperature, the moisture of the air, or the nutrition they 

 derive from the soil, the animals living on these plants will 



