202° THE DOCTRINE OF DESCENT. 
If the concatenation of the series by direct derivation 
and heredity be disallowed, it is absolutely inconceiv- 
able why the supreme creative Power, Nature, or the 
personal God, should have bound all higher animals to 
the same common stages of early development, and here- 
by exposed them to such manifold purposeless arrange- 
ments and great dangers. Of the millions of young 
oysters which annually escape from the egg, the majo- 
rity perish under the disadvantages of external condi- 
tions, because the oyster has not yet divested itself of 
the ancient heirloom of the roving navicula. It has 
been able to compete successfully in the struggle for 
existence, only because, like most of its congeners, it is 
enormously prolific. This may be understood ; but that 
a personal Creator, merely on principle, in order to keep 
the oyster within the type of development, should have 
endowed it with the phase of the navicula, in this case 
so extremely unpractical, can be accepted, like much 
other nonsense, only as matter of faith. 
If accordance in the outlines of development has 
generally shown itself derivable from similarity of de- 
scent, we may now proceed to the explanation of those 
phenomena of development known to us as hetero- 
genesis and metamorphosis. In these, the historical 
stages of development of whole classes and orders are 
inherited in the development of the individual; a pro- 
position which is merely the corollary and application of 
what has been already intimated. In no class is there 
such a profusion of the phenomena of heterogenesis, 
readily submitting to explanation, as in that of the 
Meduse. We have already (p. 43) become acquainted 
with the origin of the Cladonema from the polype-like 
