424 THKORIES OF EVOLUTION 



carry pollen from one clover plant to another, and the 

 flowers thus fertilized bear seeds. Bumble bees are de- 

 stroyed by mice, which in turn are eaten by cats. The 

 more cats the fewer mice, and the greater number of bees 

 and clover seeds. Earthworms, by tunneling the soil and 

 carrying organic food from the surface into their bur- 

 rows, are highly important agents in loosening up and 

 fertilizing the soil. These animals, therefore, are an im- 

 portant factor in determining the size of crops and the 

 number of human inhabitants in the region. The history 

 of the rabbit in Australia, the potato beetle in Ireland, 

 the codling moth, scale bugs, the water hyacinth, thistle, 

 and of many other animal and plant pests, also illustrate 

 how the balance of nature may be seriously destroyed 

 by insignificant causes. 



Variation and Heredity. — While it is true that 

 organisms tend to resemble their parents, it is equally 

 certain that they are rarely if ever their exact counter- 

 parts. In the human race, for example, there often are 

 striking family resemblances and yet invariably individ- 

 ual differences in form and structure can be shown to 

 exist. In some instances this unlikeness or variation is 

 the result of exercise or some direct effect of the environ- 

 ment. An abundance or lack of nourishment, the exer- 

 cise of a muscle, the effect of sunlight, and many other 

 agencies may produce marked changes in an animal or 

 plant during the course of its lifetime. Such effects, so 

 far as is known, are not transmitted to the offspring; they 

 are non-heritable variations or modifications. On the 

 other hand there are differences of color, form, proportion, 

 size, and thousands of other characters, known as muta- 

 tions, which are transmitted from generation to genera- 

 tion. The part w^hich the germ cells play in this process 

 was entirely unknown in Darwin's time. Since his day 

 a groat amount of information has accumulated, and the 

 method of the transmission of heritable variations is 

 almost as well known as the structure of animals and 



