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Importance of Anatomy 
The first step in our morphological study of human beings is 
to obtain an accurate and adequate knowledge of the forms of 
the individual objects which present themselves to us in our con- 
templation of the animal and vegetable kingdoms. For such 
knowledge, however, much more is needed than an acquaintance 
with their external figure. We must subject them to a searching 
scrutiny ; we must make ourselves familiar with their anatomy, 
which involves not only a knowledge of the forms and disposition 
of their organs, internal as well as external, but of their histo- 
logy, or the microscopic structure of the tissues of which these 
organs, are composed, Histology is nothing more than Anatomy 
carried to its extreme term, to that point where it meets with the 
Morphological Unit, the ultimate element of form, and the 
simplest combinations of this out of which all the organs in the 
living body are built pp. 
Among the higher animals Anatomy, in the ordinary sense of 
the word, is sufficiently distinct from Histology to admit of sepa- 
rate study ; but in the lower animals and in plants the two 
become confounded at so many points as to render their sepa- 
rate study often impracticable. 
Now the great prominence given to Anatomy is one of the 
points which most eminently distinguish the modern schools of 
Biology. 
Development 
Another order of morphological facts of scarcely less impor- 
tance than those obtained from anatomical study is that derived 
from the changes of form which the individual experiences during 
the course of its life. We know that every organised being com- 
mences existence as a simple sphere of protoplasm, and that from 
this condition of extreme generalisation all but the very lowest 
pass through phases of higher and higher specialisation acquiring 
new parts and differentiating new tissues. The sum of these 
changes constitutes the development of the organisms, and no 
series of facts is more full of significance in its bearing on Biolo- 
gical Science than that which is derived from the philosophical 
study of Development. 
Classification an Expression of Affinities 
Hitherto we have been considering the individual organism 
without any direct reference to others, But the requirements of 
the biological method can be satisfied only by a comparison cf 
the various organisms one with the other. Now the grounds of 
such comparison may be various, but what we are at present 
concerned with will be found in anatomical structure and in de- 
yelopmental changes ; and in each of these directions facts of 
the highest order and of great significance become apparent. 
By a carefully regulated comparison of one organism with 
another, we discover the resemblances as well as the differences 
betwee them. If these resemblances be strong, and occur 
in important points of structure or development, we assert that 
there is an affinity between the compared organisms, and we 
assume that the closeness of the affinity varies directly with the 
closeness of the resemblance. 
It is on the determination of these affinities that all philoso- 
phic classification of animals and plants must be based. A 
philosophical classification of organised beings aims at being a 
succinct statement of the affinities between the objects so classi- 
fied, these affini ies being at the same time so set forth as to have 
their various degrees of closeness and remoteness indicated in 
the classification, 
Affinities have long been recognised as the grounds of a natural 
biological classification, but it is only quite lately that a new sig- 
nificance has been given to them by the assumption that they 
may indicate something more than simple agreement with a 
common plan—that they may be derived by inheritance from a 
common ancestral form, and that they therefore afford evidence 
a a true blood relationship between the organisms presenting 
them. 
The recognition of this relationship is the basis of what is 
known as the Descent Theory. No one doubts that the resem- 
blances we notice among the members of such small groups as 
those we name species are derived by inheritance from a common 
ancestor, and the Descent Theory is simply the extension to the 
larger groups of this same idea of relationship. 
If this be a true principle, then biological classification becomes 
an exposition of family relationship—a genealogical tree in which 
the stem and branches indicate various degrees of relationship 
and direct and collateral lines of descent. It is this conception 
NATURE 
_out somie other evidence of their affinities no one would think of 
have two totally distinct types of structure ; that while in one 
[ Sept. 18, 1873 
which takes classification out of the domain fof the purely Mor- 
phological. 
Affinity determined by the Study of Anatomy and Development 
From what has just been said it follows that it is mainly by a 
comparison of organisms in their anatomical and developmental 
characters that their affinities are discoverable. The structure 
of an organism will in by far the greater number of cases be suf- 
ficient to indicate its true affinity, buf it sometimes happens that 
certain members of a group depart in their structure so widely 
from the characters of the type to which they belong, that with- 
assigning them to it. 
ment. 
An example or two will serve to make the subject clear, and 
we shall first take one from a case where, without a knowledge 
of anatomical structure, we should easily go astray in our attempts 
to assign to the forms under examination their true place in the 
classification. ; 
If we search our coasts at low water we shall be sure to meet 
This evidence is afforded by develop- 
with certain plant-like animals spreading over the rocks or rooted — 
to the fronds of sea-weeds, all of which present so close a resem- 
blance to one another as to have led to their being brought 
together into a sinzle group to which, under the name of 
‘* Polypes,” a definite place was assigned in the classification of 
the animal kingdom. : 
They are all composite animals consisting of an association of 
buds or zooids, which remain organically united to one another, 
and. give to the whole assemblage the appearance in many cases 
of a little branching tree. Every bud carries a delicate transpa- 
rent cup of chitine within which is contained the principal part 
of the animal, and from which this has the power of spontane- 
ously protruding itself ; and when thus protruded it will be seen 
to present a beautiful crown of tentacles surrounding a mouth 
through which food is taken into a stomach. As long as no 
danger threatens, the little animal will continue displayed with 
its beautiful corona of tentacles expanded ; but touch it ever so 
lightly, and it will instantly close up its tentacles, retract its 
whole body, and take refuge in the recesses of the protecting cup. 
So far then there is a complete agreement between the animals 
which have been thus associated under the designation of Polypes, 
and in all that concerns their external form no one point can be 
adduced in opposition to the justice of this association. When, 
however, we pass below the surface and bring the microscope 
and dissecting needle to bear on their internal organisation, we 
find that among the animals thus formed so apparently alike, we 
the mouth leads into a simple excavation of the body on which 
devolves the whole of the functions which represent digestion, 
in the other there is a complete alimentary tract entirely shut oft 
from the proper cavity of the body and consisting of distinetly 
differentiated cesophagus, stomach, and intestine ; while in the 
one the muscular system consists of an indistinct layer of fibres 
intimately united in its whole extent with the body walls, in the 
other there are distinctly differentiated free bundles of muscles 
for the purpose of effecting special motions in the economy of 
the animal ; while in the one no differentiated nervous system 
can be detected, in the other there is a distinct nervous ganglion 
with nervous filaments. In fact the two forms are shown by a 
study of their anatomical structure to belong to two entirely dif- 
ferent primary divisions of the animal kingdom ; for while the 
one has a close affinity with the little fresh-water Hydra, and is 
therefore referred to the Hydroida among the sub-kingdom Ceel- 
lenterata, the other is referable to the group of the Polyzoa ; it 
has its immediate affinities with the Ascidians, and belongs to the 
sub-kingdom of the Mollusca. 
We shall next take an example in which the study of develop- 
ment rather than of anatomy affords the clue to the true affinities 
of the organism. 
Attached to the abdomen of various crabs may often be seen 
certain soft fleshy sacs to which the name of Saccu/ina has been 
given. They hold their place by means of a branching root-like 
extension which penetrates the abdomen of the crab and winds ~ 
itself round its intestine or dives into its liver, within which its 
fibres ramify like the roots of a tree. 
Now the question at once presents itself: what position in the 
animal kingdom are we to assign to this immoveably rooted sac 
destitute of mouth and of almost every other organ with which 
we are in the habit of associating the structure of an animal ? 
Anatomy will here be powerless in helping us to arrive ata 
a 
