And hence follows the permanent preservation or speedy
destruction of the plants and animals themselves. But we are bound to
look not only to their effects but to their causes, if the theory is to
be completed. And then we cannot fail to see that these variations in
the progeny cannot be due to something in the progenitors, or otherwise
the variations would be all alike, which they certainly are not. They
must, therefore, be due to external circumstances. These slight
variations are produced by the action of the surroundings, by the food,
by the temperature, by the various accidents of life in the progenitors.
Now, when we see this, we see also how gravely it modifies the
conclusions which we have to draw concerning the ancestry of any species
now existing. Let us take, for instance, the great order of vertebrate
animals. At first sight the Darwinian theory seems to indicate that all
these animals are descended from one pair or one individual, and that
their unity of construction is due to that fact; but if we go back in
thought to the time at which the special peculiarities were introduced
which really constituted the order and separated it from other animals,
we see that it is by no means clear that it originated with one pair or
with one individual, and that, on the contrary, the probabilities are
the other way.
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