176 GENERAL ZOOLOGY 



mutants of the hornless condition have arisen from pure 

 stock within recent times. 



From the foregoing paragraphs we see that the inherent 

 tendency to vary, whether slightly or conspicuously, has fur- 

 nished the material with which man has worked to produce 

 his various breeds of domestic animals. By selection man 

 has obtained forms so widely different from each other that 

 if they occurred in nature we should not hesitate to call them 

 distinct species. Yet we know enough of their histories to 

 know that one breed of horses or of cattle has been devel- 

 oped from another. This gives us the idea that animal 

 forms are plastic and may change, producing new breeds 

 or varieties. Heredity operates on those characters wherein 

 the individual differs from others of its kind just as well as 

 on those characters which are borne by all members of the 

 race or breed. By the method of artificial selection man 

 makes the two contrasting forces of heredity and variation 

 work together to produce new races of animals. 



Menders Law of Heredity. Man has long sought for a 

 means of explaining how heredity operates. Since the open- 

 ing of the twentieth century much progress has been made 

 toward an understanding of laws, or principles, governing 

 heredity. Studies have been made on many different kinds 

 of animals representing most of the important "groups of 

 the animal kingdom. Out of these experiments there have 

 come definite laws, by means of which it is possible to pre- 

 dict with reasonable certainty what kind of offspring will 

 be produced by two individuals. Although many other laws 

 and principles are involved in the study of heredity, the 

 real explanations started with the work of Gregor Mendel. 

 He was an Austrian monk who experimented with garden 

 peas, and in 1865 he communicated his discoveries to the 

 Society of Naturalists at Briinn. These have since been 

 described as "the greatest discovery in biology since Dar- 



