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SCIENCE. 



ism arose as the result of philosophical speculation, or as ' 

 the result of imagination personifying the unity of external j 

 Nature. But if this were possible, then it would follow that ! 

 while a perception of the unit} - 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 certain 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 besel special subjects of inquiry and special 

 methods of research. The division of labor necessarily be- 

 comes more and more minute with the complication "f the 



work which is to Ijc done, and blanches out into a thousand 



channels of inquiry, each of which finds its natural termi- 

 nation in tlic ascertainment of a special seriesof relations. 

 The chemist is engaged with the elementary combinations 



of matter, and finds a unity of composition among tilings 

 which in all other aspects are totally diverse. The anato- 

 mist is i on' eined will) structure, and separates widely be- 

 tween tilings which may nevertheless be identical in chemi- 

 omposition. The physiologist is concerned with func- 

 tion; and, finding the same olfiees performed by a vast 

 vanity 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 truth 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 tin unity of the conditions which prevail among the 



worlds. Th other; Light — that sweet and heavenly 



i nger whe I) comes to us from the depths of Space, 



us all we know of other worlds, and giving us all that 



•wn. And there is one 

 condition of uni I h is not revealed by 



rravitation, Fo i respect to gravitation, although we have 

 an idea of ha 10 idea of the method, of its 



operation. We know with prei ision the numerical rules 

 i.eys, 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 ol the measure, but of the mode of its 

 operation. In one sense, of course, Light is a mere sensa- 

 ti ourselves. Hut when we speak of it as an external 



thin M ol that sensation. In this 



sense, Light ia a wave or an undulatory vibration, and such 

 vibrations can only ' m a medium which, 



however thin, must he material. Light, therefore, reveals 

 to us the fact that we are united with the most distant 



