Lectures and Essays | Page 9

which is also common to all of them; that is to
say, the Lobster, the Spider, and the Centipede, have a common plan
running through their whole arrangement, in just the same way that the
Horse, the Dog, and the Porpoise assimilate to each other.
Yet other creatures--whelks, cuttlefishes, oysters, snails, and all their
tribe ("Mollusca")--resemble one another in the same way, but differ
from both "Vertebrata" and "Annulosa"; and the like is true of the
animals called "Coelenterata" (Polypes) and "Protozoa" (animalcules
and sponges).
Now, by pursuing this sort of comparison, naturalists have arrived at
the conviction that there are,--some think five, and some seven,--but
certainly not more than the latter number--and perhaps it is simpler to
assume five--distinct plans or constructions in the whole of the animal
world; and that the hundreds of thousands of species of creatures on the
surface of the earth, are all reducible to those five, or, at most, seven,
plans of organization.

But can we go no further than that? When one has got so far, one is
tempted to go on a step and inquire whether we cannot go back yet
further and bring down the whole to modifications of one primordial
unit. The anatomist cannot do this; but if he call to his aid the study of
development, he can do it. For we shall find that, distinct as those plans
are, whether it be a porpoise or man, or lobster, or any of those other
kinds I have mentioned, every one begins its existence with one and the
same primitive form,--that of the egg, consisting, as we have seen, of a
nitrogenous substance, having a small particle or nucleus in the centre
of it. Furthermore, the earlier changes of each are substantially the
same. And it is in this that lies that true "unity of organization" of the
animal kingdom which has been guessed at and fancied for many years;
but which it has been left to the present time to be demonstrated by the
careful study of development. But is it possible to go another step
further still, and to show that in the same way the whole of the organic
world is reducible to one primitive condition of form? Is there among
the plants the same primitive form of organization, and is that identical
with that of the animal kingdom? The reply to that question, too, is not
uncertain or doubtful. It is now proved that every plant begins its
existence under the same form; that is to say, in that of a cell--a particle
of nitrogenous matter having substantially the same conditions. So that
if you trace back the oak to its first germ, or a man, or a horse, or
lobster, or oyster, or any other animal you choose to name, you shall
find each and all of these commencing their existence in forms
essentially similar to each other: and, furthermore, that the first
processes of growth, and many of the subsequent modifications, are
essentially the same in principle in almost all.
In conclusion, let me, in a few words, recapitulate the positions which I
have laid down. And you must understand that I have not been talking
mere theory; I have been speaking of matters which are as plainly
demonstrable as the commonest propositions of Euclid--of facts that
must form the basis of all speculations and beliefs in Biological science.
We have gradually traced down all organic forms, or, in other words,
we have analyzed the present condition of animated nature, until we
found that each species took its origin in a form similar to that under
which all the others commence their existence. We have found the
whole of the vast array of living forms, with which we are surrounded,

constantly growing, increasing, decaying and disappearing; the animal
constantly attracting, modifying, and applying to its sustenance the
matter of the vegetable kingdom, which derived its support from the
absorption and conversion of inorganic matter. And so constant and
universal is this absorption, waste, and reproduction, that it may be said
with perfect certainty that there is left in no one of our bodies at the
present moment a millionth part of the matter of which they were
originally formed! We have seen, again, that not only is the living
matter derived from the inorganic world, but that the forces of that
matter are all of them correlative with and convertible into those of
inorganic nature.
This, for our present purposes, is the best view of the present condition
of organic nature which I can lay before you: it gives you the great
outlines of a vast picture, which you must fill up by your own study.
In the next lecture I shall endeavour in the same way to go back into the
past, and to sketch