472 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Illustrations of Harmonic Relations.— The com- 

 plexity of the harmo'nic relations of all members of a 

 group of organisms is such that if one element is altered 

 a wave of change, often of the most unexpected kind, 

 is propagated through the group until a new harmonic 

 relation is established. It was Darwin who first drew 

 attention to the relation of cats to the flourishing of 

 clover. The fertilization of clover flowers is dependent 

 on the presence of bumble bees, but the nests of these are 

 destroyed by field mice, and field mice are destroyed by 

 cats. Professor Morgan somewhat humorously extends 

 the complex relation by adding that cats are cherished by 

 old maids. Thus the presence of old maids is favorable 

 to the growth of clover. The most unexpected results 

 often come from interference with these natural rela- 

 tions. Farmers to protect crops destroy birds, and in- 

 sects injurious to crops increase. Sportsmen introduce 

 English rabbits into Australia and New Zealand, and 

 the governments of those countries have spent millions 

 of pounds in vain attempts to destroy them. 



In a word, every species in order to continue to exist 

 must be in harmonic relation with the environment both 

 physical and organic. Now, the physical environment 

 consists of soil, climate, and geographical ■ barriers. 

 Among these the simplest and the most universal is 

 temperature. We will therefore speak first of temperature 

 regions. And among organisms the simplest in their re- 

 lations are plants. Therefore we speak first of all of 



Botanical Temperature Regions. — As we travel 

 from equator to pole we pass through successive zones 

 of temperature ranging from 8o° to o° F. These zones 

 are characterized predominantly by different groups of 

 plants. We have first a region of palms and tree ferns, 

 corresponding with the intertropical zone; then a region 

 of evergreen hard-wood trees corresponding with the 



