Lack of space compels us to leave unnoticed most of the exceedingly
valuable suggestions of Naegeli's brilliant work.
It is still less possible to do any justice in a few words to
Weismann's theory. Into its various modifications, as it has grown
from year to year, we have no time to enter. And we must confine
ourselves to his views of variation and heredity.
In studying protozoa we noticed that they reproduced by fission,
each adult individual dividing into two young ones. There is
therefore no old parent left to die. Natural death does not occur
here, only death by violence or unfavorable conditions. The protozoa
are immortal, not in the sense of the endless persistence of the
individual, but of the absence of death. Heredity is here easily
comprehensible, for one-half, or less frequently a smaller fraction,
of the substance of the parent goes to form the new individual.
There is direct continuity of substance from generation to
generation.
But in volvox a change has taken place. The fertilized egg-cell,
formed by the union of egg and spermatozoon, is a single cell, like
the individual resulting from the conjugation or fusion of two
protozoa. But in the many-celled individual, which develops out of
the fertilized egg, there are two kinds of cells. 1. There are other
egg-cells, like the first, each one of which can, under favorable
conditions, develop into a multicellular individual like the
parent.
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