90 ANIMAL LIFE 
when the earlier stages of one species correspond pretty 
closely with the early stages of another, we have a good 
basis for making up our minds about relationship between 
the two species. But it is certainly not obvious why we 
should have a similarity among the younger stages of dif- 
ferent animals and no correspondence among the older 
stages of more recent animals with the younger stages of 
more ancient ones. But on the other hand it is certainly 
true that a too specific application of the broad generaliza- 
tion that ontogeny repeats phylogeny has led to numerous 
errors of interpreting genealogic relationship. 
56. Metamorphosis.— While a young robin when it hatches 
from the egg or a young kitten at birth resembles its par- 
ents, a young star-fish or a young crab or a young butterfly 
when hatched does not at all resemble its parents. And 
while the young robin after hatching becomes a fully grown 
robin simply by growing larger and undergoing compara- 
tively slight developmental changes, the young star-fish or 
young butterfly not only grows larger, but undergoes some 
very striking developmental changes; the body changes 
very much in appearance. Marked changes in the body of 
an animal during post-embryonic or larval development 
constitute what is called metamorphic development, or the 
animal is said to undergo or to show metamorphosis in its 
development. Metamorphosis is one of the most interest- 
ing features in the life history or development of animals, 
and it can be, at least as far as its external aspects are con- 
cerned, very readily observed and studied. 
57. Metamorphosis among insects,— All the butterflies and 
moths show metamorphosis in their development. So do 
many other insects, as the ants, bees, and wasps, and all the 
flies and beetles. On the other hand, many insects do not 
show metamorphosis, but, like the birds, are hatched from 
the egg in a condition plainly resembling the parents. A 
grasshopper (Fig. 42) is a convenient example of an insect 
without metamorphosis, or rather, as there are, after all, 
