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Heredity.--The phenomena of heredity are among the most interesting
of biological studies. It is a matter of common observation that a child
looks like its parents. It even happens that a child resembles an uncle
or a grandparent more nearly than either parent. The same peculiarities
are often seen in animals.
The cause of this resemblance of offspring to parents and ancestors
has been made a subject of careful study by scientific men. We shall
present the most recent theory adopted, which, although it be but a
theory, presents such an array of facts in its support, and explains
the phenomena in question so admirably, that it must be regarded as
something more than a plausible hypothesis. It is the conception of
one of the most distinguished scientists of the age. The theory is known
as the doctrine of _pangenesis_, and is essentially as follows:--
It is a fact well known to physiologists that every part of the living
body is made up of cellular elements which have the power to reproduce
themselves in the individual, thus repairing the damage resulting from
waste and injury. Each cell produces cells like itself. It is further
known that there are found in the body numerous central points of growth.
In every group of cells is found a central cell from which the others
originated, and which determines the form of their growth. Every minute
structure possesses such a center. A simple proof of this fact is found
in the experiment in which the spur of a cock was grafted upon the ear
of an ox. It lived in this novel situation eight years, attaining the
length of nine inches, and nearly a pound in weight. A tooth has been
made to grow upon the comb of a cock in a similar manner. The tail of
a pig survived the operation of transplanting from its proper position
to the back of the animal, and retained its sensibility. Numerous other
similar illustrations might be given.
The doctrine of pangenesis supposes that these centers of nutrition
form and throw off not only cells like themselves, but very minute
granules, called gemmules, each of which is capable, under suitable
circumstances, of developing into a cell like its parent.
These minute granules are scattered through the system in great numbers.
The essential organs of generation, the testicles in the male and the
ovaries in the female, perform the task of collecting these gemmules
and forming them into sets, each of which constitutes a reproductive
element, and contains, in rudimentary form, a representative of every
part of the individual, including the most minute peculiarities. Even
more than this: It is supposed that each ovum and each zoosperm contains
not only the gemmules necessary to reproduce the individuals who
produced them, but also a number of gemmules which have been transmitted
from the individuals' ancestors.
If this theory be true,--and we can see no sound objection to it,--it
is easy to understand all the problems of heredity. The gemmules must
be very small indeed, but it may be suggested that the molecules of
matter are smaller still, so this fact is no objection to the theory.
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