49i 
embryology gives an equally emphatic proof that all 
living organisms have been gradually developed. There 
are, so far as I can see, only three conceivable ways in 
which the fertilised ovum can have become an organic being. 
A perfect microscopic organism may have been formed as 
the first magical process of reproduction, and it may 
simply have grown—i.e., increased in dimensions until the 
time of birth; or there may have been a gradual formation 
of an organism, the outlines being laid down from the first 
and filled in gradually as the process of development went 
on, or that might be which actually is, and which perhaps 
would have seemed least probable from a priori con 
siderations—the fecundated ovum might assume the forms 
of various living beings, passing from the simpler to the 
more complex. The first two of these processes would 
have been perfectly in harmony with a belief in the fixity 
of species, for the homunculus formed at the first, and the 
organism gradually developed into an ultimate complexity 
on lines laid down from the first, would simply have told 
us how a given species had power to reproduce its own 
likeness from generation to generation. But from the 
demonstrated facts of embryology there is but one reason 
able inference, and that is that the development of the 
individual organism reproduces the evolution of the race 
to which it belongs, from the lowest to the highest types 
of organism. This inference would be unassailable, even 
if it stood alone and there were no confirmatory argument; 
but the geological succession of animals and plants tends 
to establish the same principle—namely, an advance from 
the simpler to the more complex form, and the tree-like 
form into which the true classification resolves itself is 
in accordance with the argument from embryology 
and paleontological sequence. Any argument adduced 
for the progressive development of a lower into a higher