continues to deal with matters alien to the organism. I must also refrain from dwelling at length on the fact already adverted to, that the intermediate derived layer, or mesoblast, which was at the outset completely internal, originates those structures which ever remain completely internal, and have no communication with the environment save through the structures developed from the other two: an antithesis which has great significance.

Here, instead of dwelling on these details, it will be better to draw attention to the most general aspect of the facts. Whatever may be the course of subsequent changes, the first change is the formation of a superficial layer or blastoderm; and by whatever series of transformations the adult structure is reached, it is from the blastoderm that all the organs forming the adult originate. Why

this marvellous fact?

Meaning is given to it if we go back to the first stage in which Protozoa, having by repeated fissions formed a cluster, then arranged themselves into a hollow sphere, as do the protophytes forming a Volvox. Originally alike all over its surface, the hollow sphere of ciliated units thus formed, would, if not quite spherical, assume a constant attitude when moving through the water; and hence one part of the spheroid would more frequently than the rest come in contact with nutritive matters to be taken in. A division of labour resulting from such a variation being advantageous, and tending therefore to increase in descendants, would end in a differentiation like that shown in the gemmules of various low types of Metazoa, which, ovate in shape, are ciliated over one part of the surface only. There would arise a form in which the cilium-bearing units effected locomotion and aeration; while on the others, assuming an amœbalike character, devolved the function of absorbing food: a primordial specialisation variously indicated by evidence.20 Just noting that an ancestral origin of this kind is implied by the fact that in low types of Metazoa a hollow sphere of cells is the form first assumed by the unfolding embryo, I draw attention to the point here of chief interest; namely, that the primary differentiation of this hollow sphere is in such case determined by a difference in the converse of its parts with the medium and its contents; and that the subsequent invagination arises by a continuance of this differential converse.

Even neglecting this first stage and commencing with the next, in which a gastrula' has been produced by the permanent introversion of one portion of the surface of the hollow sphere, it will suffice if we consider what must thereafter have happened. That which continued to be the outer surface was the part which from time to time touched quiescent masses and occasionally received the collisions consequent on its own motions or the motions of other things. It was the part to receive the sound-vibrations occasionally propagated 20 See Balfour, vol. i. p. 149, and vol. ii. pp. 343-4.

through the water; the part to be affected more strongly than any other by those variations in the amounts of light caused by the passing of small bodies close to it; and the part which met those diffused molecules constituting odours. That is to say, from the beginning the surface was the part on which there fell the various influences pervading the environment, the part by which there was received those impressions from the environment serving for the guidance of actions, and the part which had to bear the mechanical re-actions consequent upon such actions. Necessarily, therefore, the surface was the part in which were initiated the various instrumentalities for carrying on intercourse with the environment. To suppose otherwise is to suppose that such instrumentalities arose internally where they could neither be operated on by surrounding agencies nor operate on them,-where the differentiating forces did not come into play, and the differentiated structures had nothing to do; and it is to suppose that meanwhile the parts directly exposed to the differentiating forces remained unchanged. Clearly, then, organisation could not but begin on the surface; and having thus begun, its subsequent course could not but be determined by its superficial origin. And hence these remarkable facts showing us that individual evolution is accomplished by successive in-foldings and in-growings. Doubtless natural selection soon came into action, as, for example, in the removal of the rudimentary nervous centres from the surface; since an individual in which they were a little more deeply seated would be less likely to be incapacitated by injury of them. And so in multitudinous other ways. But nevertheless, as we here see, natural selection could operate only under subjection. It could do no more than take advantage of those structural changes which the medium and its contents initiated.

See, then, how large has been the part played by this primordial factor. Had it done no more than give to Protozoa and Protophyta that cell-form which characterises them-had it done no more than entail the cellular composition which is so remarkable a trait of Metazoa and Metaphyta-had it done no more than cause the repetition in all visible animals and plants of that primary differentiation of outer from inner which it first wrought in animals and plants invisible to the naked eye; it would have done much towards giving to organisms of all kinds certain leading traits. But it has done more than this. By causing the first differentiations of those clusters of units out of which visible animals in general arose, it fixed the starting place for organisation, and therefore determined the course of organisation; and doing this, gave indelible traits to embryonic transformations and to adult structures.

Though mainly carried on after the inductive method, the argument at the close of the foregoing section has passed into the

deductive. Here let us follow for a space the deductive method pure and simple. Doubtless, in biology à priori reasoning is dangerous; but there can be no danger in considering whether its results coincide with those reached by reasoning à posteriori.

Biologists in general agree that in the present state of the world, no such thing happens as the rise of a living creature out of nonliving matter. They do not deny, however, that at a remote period in the past, when the temperature of the Earth's surface was much higher than at present, and other physical conditions were unlike those we know, inorganic matter, through successive complications, gave origin to organic matter. So many substances once supposed to belong exclusively to living bodies, have now been formed artificially, that men of science scarcely question the conclusion that there are conditions under which, by yet another step of composition, quaternary compounds of lower types pass into those of highest types. That there once took place a gradual divergence of the organic from the inorganic, is, indeed, a necessary implication of the hypothesis of Evolution, taken as a whole; and if we accept it as a whole, we must put to ourselves the question-What were the early stages of progress which followed, after the most complex form of matter had arisen out of forms of matter a degree less complex?

At first, protoplasm could have had no proclivities to one or other arrangement of parts; unless, indeed, a purely mechanical proclivity towards a spherical form when suspended in a liquid. At the outset it must have been passive. In respect of its passivity, primitive organic matter must have been like inorganic matter. No such thing as spontaneous variation could have occurred in it; for variation implies some habitual course of change from which it is a divergence, and is therefore excluded where there is no habitual course of change. In the absence of that cyclical series of metamorphoses which even the simplest living thing now shows us, as a result of its inherited constitution, there could be no point d'appui for natural selection. How, then, did organic evolution begin?

If a primitive mass of organic matter was like a mass of inorganic matter in respect of its passivity, and differed only in respect of its greater changeableness; then we must infer that its first changes conformed to the same general law as do the changes of an inorganic mass. The instability of the homogeneous is a universal principle. In all cases the homogeneous tends to pass into the heterogeneous, and the less heterogeneous into the more heterogeneous. In the primordial units of protoplasm, then, the step with which evolution commenced must have been the passage from a state of complete likeness throughout the mass to a state in which there existed some unlikeness. Further, the cause of this step in one of these portions of organic matter, as in any portion of inorganic matter, must have been the different exposure of its parts to incident forces. What

incident forces? Those of its medium or environment. Which were the parts thus differently exposed? Necessarily the outside and the inside. Inevitably, then, alike in the organic aggregate and the inorganic aggregate (supposing it to have coherence enough to maintain constant relative positions among its parts), the first fall from homogeneity to heterogeneity must always have been the differentiation of the external surface from the internal contents. No matter whether the modification was physical or chemical, one of composition or of decomposition, it comes within the same generalisation. The direct action of the medium was the primordial factor of organic evolution.

In his article on Evolution in the Encyclopædia Britannica, Professor Huxley writes as follows:

'How far "natural selection" suffices for the production of species remains to be Few can doubt that, if not the whole cause, it is a very important factor in that operation. . . .

seen.

'On the evidence of paleontology, the evolution of many existing forms of animal life from their predecessors is no longer an hypothesis, but an historical fact; it is only the nature of the physiological factors to which that evolution is due which is still open to discussion.'

With these passages I may fitly join a remark made in the admirable address Prof. Huxley delivered before unveiling the statue of Mr. Darwin in the Museum at South Kensington. Deprecating the supposition that an authoritative sanction was given by the ceremony to the current ideas concerning organic evolution, he said that 'science commits suicide when it adopts a creed.'

Along with larger motives, one motive which has joined in prompting the foregoing articles, has been the desire to point out that already among biologists, the beliefs concerning the origin of species have assumed too much the character of a creed; and that while becoming settled they have been narrowed. So far from further broadening that broader view which Mr. Darwin reached as he grew older, his followers appear to have retrograded towards a more restricted view than he ever expressed. Thus there seems occasion for recognising the warning uttered by Prof. Huxley, as not uncalled for.

Whatever may be thought of the arguments and conclusions set forth in this article and the preceding one, they will perhaps serve to show that it is as yet far too soon to close the inquiry concerning the causes of organic evolution.

HERBERT SPENCER.

RAILWAY TRAFFIC AND CHARGES.

Or the provisions which this Bill contains, those which have caused most apprehension to the Railway interest, and which will encounter most opposition, are undoubtedly the provisions by which power is given to the Board of Trade, subject to the control of Parliament, to revise rates. It is in respect of these provisions that such exciting terms' as 'confiscation' and 'breach of faith' have been used: expressions which are apparently not due to hallucinations which exist only in the minds of those who lead the railway world, for the Joint Committee on Railway Amalgamation in 1872 shared by anticipation their apprehensions. Discussing the question of immediate or periodical revision of rates, their report says (p. xxxv), ' On what principles is it to be performed, and by whom? If no rule is laid down to guide the revisers, the power of revision will amount to a power to confiscate the property of the companies. It is not likely that Parliament would attempt to exercise any such power itself, still less that it would confer such a power on any subordinate authority.'

The report goes on to discuss the rules which, it was suggested, might be laid down to guide the revisers, and shows that they are impracticable; but as no rule is laid down in Mr. Mundella's Bill, it is unnecessary to discuss them, and the fact remains that, in the opinion of that committee, this Bill provides a power to confiscate shareholders' property. The thought naturally occurs, what a valuable instrument would be ready to hand in this power if ever the State thought fit to purchase the railways. It would be so easy to depreciate the property which had to be purchased. Would the temptation to make some use of this power be resisted in such a case, and for how long?

Let us, however, examine the case for the Bill. Nobody, as yet, has had the courage to say that Parliament would be justified in interfering with powers of charge, which were granted to the railway companies, in return for solid national advantages, as part of their charter, unless the power to interfere had been accepted by them as part of the bargain. Accordingly, the contemplated interference is excused upon the ground that the shareholders have accepted their powers subject to a saving clause which justifies the proposed revision