96 
AMPHIBIANS 
have preserved the remains of only a small part of the forms 
of the past. Many of the records 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'riin-ment). 
80. 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; it 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 expresses itself in the 
growing plant and animal by making sure that it resembles 
its parents. Thus heredity determines that leaves of the 
right shape and size occur in the proper place or that fins 
in the fish or arms in the frog shall form in their normal posi¬ 
tion. The subject of heredity in its relation to man is pre¬ 
sented in the chapter on Human Progress in this book. 
A detailed statement of the laws of heredity is beyond the 
province of an elementary book, but it is now well estab¬ 
lished that certain traits of parent plants and animals are 
reproduced in their offspring in regular and definite amounts 
and proportions. 
81. Environment. — This word is used in two ways. 
First, it refers to general surroundings such as temperature, 
moisture, and seasons, as they vary from year to year; and 
secondly, to immediate surroundings. The frog responds 
