ifi 
CHAPTER II 
DEVELOPMENT AND META¬ 
MORPHOSIS 
HAT animals are born or hatch from eggs in 
an immature condition is such familiar natural 
history that we are likely to overlook the 
significance and consequences of the fact unless 
our attention is particularly called to them. 
This condition of immaturity makes it necessary 
that part of the free life of the organism has 
to be devoted to growth and development and 
has to be undergone in an imperfect condition, 
a condition of structure and physiology, indeed, which may be very different 
from that of the parents or of maturity. While most animals that are born 
alive re:emble the parents in most respects, always excepting that of size, 
many of those animals which hatch from eggs deposited outside the body 
of the mother issue from the egg with few indeed of the characteristics of the 
parents and may be so dissimilar from them that only our knowledge of 
the life-history of the animal enables us to recognize these young individuals 
as of the same species as the parent. The butterfly hatching as the worm¬ 
like caterpillar, and the frog as the fish-like tadpole, are the classic examples 
of this phenomenon. The mammals, our most familiar examples of animals 
which give birth to their young alive and free, nourish, for weeks or months 
before birth, the developing growing young. But with egg-laying animals 
usually only such nourishment is furnished the young as can be enclosed 
as food-yolk within the egg-shell. As a matter of fact, some young which 
hatch from eggs, as, for example, chickens, quail, etc., hatch in well- 
developed condition; and some young mammals, nourished by the mother’s 
body until birth, are in a conspicuously undeveloped state, as a young 
kangaroo or opossum. But nevertheless it is generally true that an animal 
hatched from an egg has still a larger amount of development to undergo 
before it comes to the stature and capacity of its parents than one which is 
35 
