382 BIOLOGY 



2. Food. Animals and plants are limited to territories which 

 furnish the food on which they subsist. A territory that fails 

 to produce sufficient food for any given type of animal will 

 prove an effectual barrier. 



3. Temperature. Forms of life adapted to a warm climate 

 cannot live in a cold climate, and vice versa. The temperature 

 of a territory is, therefore, a highly important factor in deter- 

 mining its inhabitants. Most animals living in cold regions will 

 not pass over the equator, and those adapted to the warm 

 equatorial climate cannot distribute themselves over the colder 

 regions. 



Enemies. Every animal and plant has its special enemies. 

 These enemies are sometimes in the form of parasites; they 

 may be larger animals and plants, or other organisms that are 

 contending for the same food. The mutual rivalries of organ- 

 isms make one of the most complex problems of biology, and one 

 that presents an endless puzzle. The introduction of any new 

 animals into an old territory may produce unexpected changes 

 in the life of the animals and plants, the newly arriving organ- 

 isms seizing the available food, or destroying the life of other 

 animals and plants, and giving rise to modifications in the 

 fauna and flora, which can never be anticipated or predicted. 

 The complexity of these relations is indicated in a famous 

 example given by Darwin. The clover crop is dependent upon 

 the bumblebees, which distribute its pollen and produce proper 

 fertilization; the number of bumblebees is dependent upon the 

 number of field mice who eat them; the field mice in turn are 

 eaten by the cats; so that in this roundabout way the number 

 of cats in a territory regulates the clover crop. 



Change of type under new conditions. The distribution of any 

 particular species of animal or plant is modified by another 

 factor of a different nature. When an animal migrates into a 

 new territory, and comes under totally different conditions as 

 to food, climate, and enemies, it is very apt to begin to change. 

 These variations from the original type may, in the new terri- 



