ANIMAL RELATIONSHIPS . 249 
fact that such important changes occur so easily during 
growth leads us to believe that adult forms may undergo 
modifications by processes as simple as these foldings, 
if time be allowed for gradual changes through many 
generations. 
We have noted the gradual disappearance of the tail 
of the tadpole as it reaches maturity. This tail is use¬ 
ful to its possessor for a time, and dwindles away when 
no longer needed. The tail, and the gills as well, 
suggest a relationship between frogs and fishes. If we 
Stage. 
Fig. 204.—Gastrula Stage. A, outer layer 
of cells; B , inner layer; C , external open¬ 
ing, or mouth; D , internal cavity. 
examine the embryo of a bird we find, at an early 
period, a well-formed tail which can be of no possible 
use to its owner. This tail disappears, as does that of 
the frog, with further growth. Even more strongly 
than in the case of the frog is relationship with tailed 
vertebrates suggested. Naturalists generally believe 
that this tail is a peculiarity inherited from a reptile-like 
or fish-like ancestor. Fossil forms like the archae¬ 
opteryx strengthen this opinion. Many other parts of 
animal bodies noticed in the process of growth are 
equally useless to their owners. These are now com¬ 
monly regarded as inheritances from ancestors to whom 
these parts were useful. Thus, the possession of gill- 
