104 Darwin, and after Darwin. 
condensation, but also in the way of excision. Many 
pages of ancestral history may be recapitulated in 
the paragraphs of embryonic development, while 
others may not be so much as mentioned. And that 
this is the true explanation of what embryologists 
term “direct” development—or of a more or less 
‘sudden leap from one phase to another, without any 
appearance of intermediate phases—is proved by the 
fact that in some cases bothdirect and indirect develop- 
ment occur within the same group of organisms, some 
genera or families having dropped out the intermediate 
phases which other genera or families retain. 
The argument from embryology must be taken to 
begin with the first beginning of individual life in the 
ovum. And, in order to understand the bearings of 
the argument in this its first stage, we must consider 
the phenomena of reproduction in the simplest form 
which these phenomena are known to present. 
The whole of the animal kingdom is divided into 
two great groups, which are called the Protozoa and 
the Metazoa. Similarly, the whole of the vegetable 
kingdom is divided into the Protophyta and the Meta- 
phyta. The characteristic feature of all the Protozoa 
and Protophyta is that the organism consists of a 
single physiological cell, while the characteristic of all 
the Metazoa and Metaphyta is that the organism 
consists of a plurality of physiological cells, variously 
modified to subserve different functions in the 
economy of the animal or plant, as the case may be. 
For the sake of brevity, I shall hereafter deal only 
with the case of animals (Protozoa and Metazoa); but 
it may throughout be understood that everything 
