182 
SCIENCE. 
ism arose as the result of philosophical speculation, or as 
the result of imagination personifying the unity of external 
Nature. But if this were possible, then it would follow that 
while a perception of the unity or the unity of Nature must 
be at least as old as the idea of one Creator, it may be a 
good deal older. Whether the two ideas were ever actually 
separated in history, it is certairr that they can be, and are, 
separated at the present time. A sense and a perception of 
the unity of Nature — strong, imaginative, and almost mys- 
tic in its character — is now prevalent among men over whom 
the idea of the personal agency of a living God has, to say 
the least, a much weaker hold. 
What, then, is this unity of Nature? Is it a fact or an 
imagination ? Is it a reality or a dream ? Is it a mere po- 
etic fancy incapable of definition ; or is it a conception 
firmly and legitimately founded on the phenomena of the 
world. 
But there is another question which comes before these. 
What do we mean by unity? In what sense can we say 
that an infinite number and a variety of things are never- 
theless one? This is an important question, because it is 
very possible to look for the unity of Nature in such a man- 
ner that, instead of extending our knowledge, or rendering 
it more clear and definite, we may rather narrow it, and 
render it more confused. It has been said that all know- 
ledge consists in the mere perception of difference. This is 
not accurate : but it is true that the perception of difference is 
the necessary foundation of all knowledge. For if it be 
possible to give any short definition of that in which essen- 
tially all knowledge consists, perhaps the nearest approach 
to such a definition would be this: that knowledge is the 
perception of relations. To know a thing and to under- 
stand it, is to know it in its relation to other things. 
But the first step in this knowledge is to know it as distin- 
guished from other things. The perception of difference 
comes before the perception of all other and higher relations. 
It is well, therefore, to remember that no increase of know- 
ledge can be acquired by a willful forgetfulness of distin- 
ctions. We may choose to call two things one, because 
we choose to look at them in one respect only, and to 
disregard them in other respects quite as obvious, and 
perhaps much more important. And thus we may create a 
unity which is purely artificial, or which represents nothing 
but a comparatively insignificant incident in the system of 
Nature. For as things may be related to each other in an 
infinite variety of ways — in form, or in size, or in substance, 
or in position, or in modes of origin, or in laws of growth, 
or in work and function — so there are an infinite number and 
variety of aspects in which unity can be traced. And these 
aspects rise in an ascending series according to the complete- 
ness of our knowledge of things, and according to the de- 
velopment of those intellectual faculties by which alone the 
higher relations between them can be perceived. For the 
perception of every relation, even that of mere physical con- 
tinuity, is purely the work of mind, and this work can only 
be performed in proportion to the materials which are sup- 
plied, and to the power of interpretation which is enjoyed. 
It is very easy to rest satisfied with the perception of the 
commoner and more obvious relations of things to each other, 
and even to be so engrossed with these as to be rendered 
altogether incapable of perceiving the finer and less palpable 
relations which constitute the higher unities of Nature. New 
relations, too, by no means obvious, but discovered by analy- 
sis, may, from the mere effect of novelty, engross atten- 
tion far beyond their real importance. Nay, more — it may 
be said, with truth, that this is a danger which, for a time at 
least, increases with the progress of science, because it 
must obviously beset special subjects of inquiry and special 
methods of research. The division of labor necessarily be- 
comes more and more minute with the complication of the 
work which is to be done, and branches out into a thousand 
channels of inquiry, each of which finds its natural termi- 
nation in the ascertainment of a special series of relations. 
The chemist is engaged with the elementary combinations 
of matter, and finds a unity of composition among things 
which in all other aspects are totally diverse. The anato- 
mist is concerned with structure, and separates widely be- 
tween things which may nevertheless be identical in chemi- 
cal composition. The physiologist is concerned with func- 
tion ; and, finding the same offices performed by a vast 
variety of structures, ranges them across all their differences 
under a single name. The comparative anatomist is con- 
cerned with the relative place or position of the parts in or- 
ganic structures ; and, although he finds the same part in 
different creatures performing widely different functions, 
he nevertheless pronounces them to be the same, and to be 
one in the homologies of an ideal archetype. But each of 
these inquirers may be satisfied with the particular unity 
which his own investigations lead him specially to observe, 
and may be blind altogether to the unity which is next 
above it. And so it may well be that the sense of unity in 
Nature, which Man has had from very early times, reflected 
in such words as the “ Universe," and in his belief in one 
God, is a higher and fuller perception of the trpth than is 
commonly attained by those who are engrossed by the labor- 
ious investigation of details. This is one of the many 
cases in which the intuitions of the mind have preceded in- 
quiry, and gone in advance of science, leaving nothing for 
systematic investigation to do, except to confirm, by formal 
proofs, that which has been already long felt and known. 
I have already indicated the sense in which the unity of 
Nature impresses itself on the intelligence of Man. It is 
in that intricate dependence of all things upon each other 
which makes them appear to be parts of one system. And 
even where the connection falls short of dependence, or of 
any visible relation, the same impression of unity is con- 
veyed in the prevalence of close and curious analogies 
which are not the less striking when the cause or the reason 
of them is unknown. 
I propose in this chapter to specify some of the signs of 
unity which the study of Nature has more definitely re- 
vealed, and consider how far they carry us. 
There is one sign of unity which, of itself, carries us 
very far indeed. It is the sign given to us in the ties by 
which this world of ours is bound to the other worlds 
around it. There is no room for fancy here. The truths 
which have been reached in this matter have been reached 
by the paths of rigorous demonstration. This earth is part 
of the vast mechanism of the heavens. The force, or 
forces, by which that mechanism is governed are forces 
which prevail not only in our own solar system, but, as 
there is reason to believe, through all Space, and are deter- 
mining, as astronomers tell us, the movement of our sun, 
with all its planets, round some distant centre, of which 
we know neither the nature nor the place. Moreover, these 
same forces are equally prevailing on the surface of this 
earth itself. The whole of its physical phenomena are 
subject to the conditions which they impose. 
If there were no other indications of unity than this, it 
would be almost enough. For the unity which is implied 
in the mechanism of the heavens is indeed a unity which is 
all-embracing and complete. The structure of our own 
bodies, with all that depends upon it, is a structure gov- 
erned by, and therefore adapted to, the same force of gravi- 
tation which has determined the form and the movement of 
myraids of worlds. Every part of the human organism is 
fitted to conditions which would all be destroyed in a 
moment if the forces of gravitation were to change or fail. 
It is, indeed, evident that a force such as this must govern 
the whole order of things in which it exists at all. Every 
other force must work, or be worked, in subordination 
to it. 
Nor is gravitation the only agency which brings home to 
us the unity of the conditions which prevail among the 
worlds. There is another : Light — that sweet and heavenly 
messenger which comes to us from the depths of Space, 
telling us all we know of other worlds, and giving us all that 
we enjoy of life and beauty on our own. And there is one 
condition of unity revealed by Light which is not revealed by 
gravitation. For, in respect to gravitation, although we have 
an idea of the measure , we have no idea of the method , of its 
operation. We know with precision the numerical rules 
which it obeys, but we know nothing whatever of the way 
in which its work is done. But in respect to Light, we have 
an idea not only of the measure, but of the mode of its 
operation. In one sense, of course, Light is a mere sensa- 
tion in ourselves. But when we speak of it as an external 
thing, we speak of the cause of that sensation. In this 
sense, Light is a wave or an undulatory vibration, and such 
vibrations can only be propagated in a medium which, 
however thin, must be material. Light, therefore, reveals 
to us the fact that we are united with the most distant 
