hese early forms
that one is able to trace the analogy they bear with the higher forms.
No rigid line can be drawn between the successive stages of growth.
And it should be borne in mind that, simple as is the life-process in
these single-celled organisms, many of them are highly differentiated
and show great complexity of structure within the narrow limits of
their size. Thus among the _protozoa_, the basis of all animal life,
we find very definite and interesting modes of behaviour, such as
seeking light and avoiding it, swimming in a spiral, approaching
certain substances and retreating from others; the organisms often,
indeed, trying one behaviour after another.[10] If we realise this it
becomes easier to understand how the higher types of life have
developed from these primitive types. Indeed, all the bodies of the
most complex animals--including ourselves--originate as simple cells,
and in the individual history of each of us divide and multiply just
as do the cells which exist independently; only in multicellular
organisms each cell must be regarded as an individual, modified to
serve a special purpose, one cell differentiated to start a lineage of
nerve cells, another a lineage of digestive cells, yet another for the
reproduction of the species, and so on, each group of cells taking on
its special use, but the power of division remaining with the modified
cell. Thus a new life is built up--a child becomes an adult, by
multiplication of these differentiated cells, repeating the original
single-cell development.

Budding, the second, and perhaps the most usual mode of asexual
propagation, may be said to mark a further step in the development of
the reproductive process. Here the mother-cell, instead of dividing
into two equal parts and at once rupturing, protrudes a small portion
of its substance, which is separated by a constriction that grows
deeper and deeper until the bulk becomes wholly detached. This small
bud then grows until it attains the size of the parent, when it, in
turn, repeats the same process. This mode of reproduction is common to
the great majority of plants. In animal life it is not confined to
single-celled organism, but takes place in certain multicellulars,
such as worms, bryozoans, and ascidians; one very interesting example
being the sea-worm (_myrianida_) which buds off a whole chain of
individuals.

Nearly allied with budding is the third stage, in which the division
is multiple and r