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ON THE STUDY OF ZOOLOGY
ON THE STUDY OF ZOOLOGY THOMAS H. HUXLEY [footnote] *A Lecture delivered at the South Kensington
Museum in 1861.
NATURAL HISTORY is the name familiarly applied to the study of the
properties of such natural bodies as minerals plants and animals; the
sciences which embody the knowledge man has acquired upon these
subjects are commonly termed Natural Sciences in contradistinction to
other so-called "physical" sciences; and those who devote themselves
especially to the pursuit of such sciences have been and are commonly
termed "Naturalists."
Linnaeus was a naturalist in this wide sense and his 'Systema Naturae'
was a work upon natural history in the broadest acceptation of the
term; in it that great methodising spirit embodied all that was known
in his time of the distinctive characters of minerals animals and
plants. But the enormous stimulus which Linnaeus gave to the
investigation of nature soon rendered it impossible that any one man
should write another 'Systema Naturae' and extremely difficult for any
one to become even a naturalist such as Linnaeus was.
Great as have been the advances made by all the three branches of
science of old included under the title of natural history there can
be no doubt that zoology and botany have grown in an enormously greater
ratio than mineralogy; and hence as I suppose the name of "natural
history" has gradually become more and more definitely attached to these
prominent divisions of the subject and by "naturalist" people have
meant more and more distinctly to imply a student of the structure and
function of living beings.
However this may be it is certain that the advance of knowledge has
gradually widened the distance between mineralogy and its old
associates while it has drawn zoology and botany closer together; so
that of late years it has been found convenient (and indeed necessary)
to associate the sciences which deal with vitality and all its
phenomena under the common head of "biology"; and the biologists have
come to repudiate any blood-relationship with their foster-brothers
the mineralogists.
Certain broad laws have a general application throughout both the animal
and the vegetable worlds but the ground common to these kingdoms of
nature is not of very wide extent and the multiplicity of details is
so great that the student of living beings finds himself obliged to
devote his attention exclusively either to the one or the other. If he
elects to study plants under any aspect we know at once what to call
him. He is a botanist and his science is botany. But if the
investigation of animal life be his choice the name generally applied
to him will vary according to the kind of animals he studies or the
particular phenomena of animal life to which he confines his
attention. If the study of man is his object he is called an
anatomist or a physiologist or an ethnologist; but if he dissects
animals or examines into the mode in which their functions are
performed he is a comparative anatomist or comparative physiologist.
If he turns his attention to fossil animals he is a palaeontologist.
If his mind is more particularly directed to the specific description
discrimination classification and distribution of animals he is
termed a zoologist.
For the purpose of the present discourse however I shall recognise
none of these titles save the last which I shall employ as the
equivalent of botanist and I shall use the term zoology as denoting
the whole doctrine of animal life in contradistinction to botany which
signifies the whole doctrine of vegetable life.
Employed in this sense zoology like botany is divisible into three
great but subordinate sciences morphology physiology and
distribution each of which may to a very great extent be studied
independently of the other.
Zoological morphology is the doctrine of animal form or structure.
Anatomy is one of its branches; development is another; while
classification is the expression of the relations which different
animals bear to one another in respect of their anatomy and their
development.
Zoological distribution is the study of animals in relation to the
terrestrial conditions which obtain now or have obtained at any
previous epoch of the earth's history.
Zoological physiology lastly is the doctrine of the functions or
actions of animals. It regards animal bodies as machines impelled by
certain forces and performing an amount of work which can be expressed
in terms of the ordinary forces of nature. The final object of
physiology is to deduce the facts of morphology on the one hand and
those of distribution on the other from the laws of the molecular
forces of matter.
Such is the scope of zoology. But if I were to content myself with the
enunciation of these dry definitions I should ill exemplify that
method of teaching this branch of physical science which it is my
chief business to-night to recommend. Let us turn away then from
abstract definitions. Let us take some concrete living thing some
animal the commoner the better and let us see how the application of
common sense and common logic to the obvious facts it presents
inevitably leads us into all these branches of zoological science.
I have before me a lobster. When I examine it what appears to be the
most striking character it presents? Why I observe that this part
which we call the tail of the lobster is made up of six distinct hard
rings and a seventh terminal piece. If I separate one of the middle
rings say the third I find it carries upon its under surface a pair
of limbs or appendages each of which consists of a stalk and two
terminal pieces. So that I can represent a transverse section of the
ring and its appendages upon the diagram board in this way.
If I now take the fourth ring I find it has the same structure and so
have the fifth and the second; so that in each of these divisions of
the tail I find parts which correspond with one another a ring and
two appendages; and in each appendage a stalk and two end pieces.
These corresponding parts are called in the technical language of
anatomy "homologous parts." The ring of the third division is the
"homologue" of the ring of the fifth the appendage of the former is
the homologue of the appendage of the latter. And as each division
exhibits corresponding parts in corresponding places we say that all
the divisions are constructed upon the same plan. But now let us
consider the sixth division. It is similar to and yet different from
the others. The ring is essentially the same as in the other divisions;
but the appendages look at first as if they were very different; and
yet when we regard them closely what do we find? A stalk and two
terminal divisions exactly as in the others but the stalk is very
short and very thick the terminal divisions are very broad and flat
and one of them is divided into two pieces.
I may say therefore that the sixth segment is like the others in plan
but that it is modified in its details.
The first segment is like the others so far as its ring is concerned
and though its appendages differ from any of those yet examined in the
simplicity of their structure parts corresponding with the stem and
one of the divisions of the appendages of the other segments can be
readily discerned in them.
Thus it appears that the lobster's tail is composed of a series of
segments which are fundamentally similar though each presents peculiar
modifications of the plan common to all. But when I turn to the
forepart of the body I see at first nothing but a great shield-like
shell called technically the "carapace" ending in front in a sharp
spine on either side of which are the curious compound eyes set upon
the ends of stout movable stalks. Behind these on the under side of
the body are two pairs of long feelers or antennae followed by six
pairs of jaws folded against one another over the mouth and five pairs
of legs the foremost of these being the great pinchers or claws of
the lobster.
It looks at first a little hopeless to attempt to find in this complex
mass a series of rings each with its pair of appendages such as I
have shown you in the abdomen and yet it is not difficult to
demonstrate their existence. Strip off the legs and you will find that
each pair is attached to a very definite segment of the under wall of
the body; but these segments instead of being the lower parts of free
rings as in the tail are such parts of rings which are all solidly
united and bound together; and the like is true of the jaws the
feelers and the eye-stalks every pair of which is borne upon its own
special segment. Thus the conclusion is gradually forced upon us that
the body of the lobster is composed of as many rings as there are pairs
of appendages namely twenty in all but that the six hindmost rings
remain free and movable while the fourteen front rings become firmly
soldered together their backs forming one continuous shield--the
carapace.
Unity of plan diversity in execution is the lesson taught by the study
of the rings of the body and the same instruction is given still more
emphatically by the appendages. If I examine the outermost jaw I find
it consists of three distinct portions an inner a middle and an
outer mounted upon a common stem; and if I compare this jaw with the
legs behind it or the jaws in front of it I find it quite easy to
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