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, 



