nd
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 Metaphyta. 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 which is said applies also to the case of
plants (Protophyta and Metaphyta).

A Protozooen (like a Protophyton) is a solitary cell, or a "unicellular
organism," while a Metazooen (like a Metaphyton) is a society of cells,
or a "multicellular organism." Now, it is only in the multicellular
organisms that there is any observable distinction of sex. In all the
unicellular organisms the phenomena of reproduction appear to be more or
less identical with those of growth. Nevertheless, as these phenomena
are here in some cases suggestively peculiar, I will consider them more
in detail.

A Protozooen is a single corpuscle of protoplasm which in different
species of Protozoa varies in size from more than one inch to less than
1/1000 of an inch in diameter. In some species there is an enveloping
cortical substance; in other species no such substance can be detected.
Again, in most species there is a nucleus, while in other species no
such differentiation of structure has hitherto been observed.
Nevertheless, from the fact that the nucleus occurs in the majority of
Protozoa, coupled with the fact that the demonstration of this body is
often a matter of extreme difficulty, not only in some of the Protozoa
where it has been but recently detected, but also in the case of certain
physiological cells elsewhere,--from these facts it is not unreasonable
to suppose that all the Protozoa possess a nucleus, whether or not it
admits of being rendered visible by histological methods thus far at our
disposal. If this is the case, we should be justified in saying,