Among certain infusoria and
other protista, they do, in fact, remain together and build up
branching colonies. At the end of each branch is situated an
infusorian (vorticella), and the whole colony represents in itself the
genealogical family tree.
In the beginning, there existed no other animal organisms than these
aggregations of similar unicellular beings, all of which reproduced
themselves. Later on, division of labor made its appearance among the
individuals of the animal colony, and it increased their dependence
upon one another, so that their individuality was to a great extent
lost, and they were no longer able to live independently of one
another.
By the development of this process, multicellular metazoa arose from
the colonies of similar protozoa, and at length culminated in the
higher animals and man.
If we examine the human body, its origin and end, in the light of
these facts, we shall see that a comparison between the simple
immortal protozoa and man leads us to the result that man himself, or
at least a part of him and that the most important, is immortal.
When we turn to the starting point of human development, we find an
egg cell and a spermatozoon, which unite and whose nuclei intermingle.
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