HEREDITY 125 



of extinct animals and plants have been destroyed by decay 

 and heat so that much that would be valuable in solving 

 the question can never be found. 



The study of the development of the frog also illustrates 

 two other general subjects, heredity (he-red'I-ty ) and en- 

 vironment (en-vl'run-ment). 



123. Heredity. — The tendency of all young animals to 

 grow and live like their parents is called heredity and may 

 be defined as the transmission of physical and mental traits 

 from parent to offspring. There is no difficulty in recog- 

 nizing the new frog as a certain kind of frog. The color 

 markings on the skin are like those of the parents; it 

 grows to about the same size; eats the same kind of food, 

 and lives in the same region. 



Every species of living thing is able to produce new 

 forms like itself, and heredity is always at work when 

 new plants and animals are being produced. Heredity is 

 best thought of as that quality of living matter which ex- 

 presses itself in the growing plant and animal by making 

 sure that it resembles its parents. Thus heredity deter- 

 mines that leaves of the right shape and size occur in the 

 proper place and that our fingers and thumbs grow on the 

 end of the arm in the usual way. 



There has been much study of the question of heredity 

 and there is much yet to be learned. However, we know- 

 that we inherit from our parents and grandparents, our 

 complexion, the color of eyes and hair, our size, our re- 

 sistance to disease, our mental traits, and many other 

 characteristics. 



In 1865 Gregor Mendel, abbot of Briinn, published t la- 

 results of experiments made with peas, which showed thai 

 crossing tall and dwarf peas resulted in all the offspring 

 being tall. But the offspring of these latter (the grand- 

 children, so to speak, of the original peas) might be cither 



