118 Darwin, and after Darwin. 
I will now pass on to consider the embryogeny of 
the Metazoa, beginning at its earliest stage in the 
fertilization of the ovum. And here it is that the 
constructive argument in favour of evolution which 
is derived from embryology may be said properly to 
commence. For it is surely in itself a most suggestive 
fact that all the Metazoa begin their life in the same 
way, or under the same form and conditions. Ommme 
vivum ex ovo. This is a formula which has now been 
found to apply throughout the whole range of the 
multicellular organisms. And seeing, as we have just 
seen, that the ovum is everywhere a single cell, the 
formula amounts to saying that, physiologically 
speaking, every Metazoén begins its life as a Pro- 
tozoén, and every Metaphyton as a Protophyton!. 
Now, if the theory of evolution is true, what should 
we expect to happen when these germ cells are fer- 
tilized, and so enter upon their severally distinct 
processes of development? Assuredly we should 
expect to find that the higher organisms pass through 
the same phases of development as the lower or- 
ganisms, up to the time when their higher characters 
begin to become apparent. If in the life-history of 
species these higher characters were gained by gradual 
improvement upon lower characters, and if the de- 
velopment of the higher individual is now a general 
recapitulation of that of its ancestral species, in studying 
this recapitulation we should expect to find the higher 
organism successively unfolding its higher characters 
from the lower ones throvgh which its ancestral species 
had previously passed. And this is just what we do 
' Even when propagated by budding, a multicellular organism has 
been ultimately derived from a germ-cell. 
