Biology Reference
In-Depth Information
that, even if there was a cause, e.g. a change in the living conditions
of the organism, the result was not necessarily uniform, variations
induced by the same cause possibly differing one from another.
In this respect, he anticipated mutation as we understand it today.
Darwin's theory of heredity, which he himself qualified as a pro-
visional hypothesis, is particularly interesting because it shows the
persistence of the pangenetic conception in the period that imme-
diately preceded the rapid development of genetics. At that time
biology had already made major progress, in particular with the
discovery of the cell which was now accepted as the basic unit of
living organisms. It is fascinating to see to what extent Darwin's
theory still resembled that of Hippocrates, despite these develop-
ments. Here in his own words is his main hypothesis.
“It is universally admitted that the cells or units of the body
increase by self-division or proliferation, retaining the same
nature, and that they ultimately become converted into the various
tissues and substances of the body. But besides this means of
increase I assume that the units throw off minute granules which
are dispersed throughout the whole system; that these, when sup-
plied with proper nutriment, multiply by self-division, and are ulti-
mately developed into units like those from which they were
originally derived. These granules may be called gemmules. They
are collected from all parts of the system to constitute the sexual
elements, and their development in the next generation forms a
new being; but they are likewise capable of transmission in a dor-
mant state to future generations and may then be developed”
(Darwin, 1868, Vol. II, pp. 369-370).
The same principle is still of emanations from each part of the
body allowing its global reproduction (Fig. 32), but Darwin's the-
ory is nevertheless more sophisticated on certain points. It supposes
that the gemmules are secreted not only in the adult stage but at
all stages of development. These unite with the cells to give them
their specific character and embryogenesis is thus guided by these
minuscule granules corresponding to each stage. Darwin was aware
