buds are formed in axils, occurred to me in this light. For an axil is an interspace, a point of separation, at which the resistance to the outgrowth of the plastic material might naturally be supposed less than at other portions of the stem. Following this clue, I perceived that the conception of gemmation in axils appeared applicable, to a large extent, to the processes in which development consists. The eye and the ear bud out in the interspaces between the primary divisions of the encephalon; the vascular lamina is formed between the two layers of the germinal membrane; the allantois insinuates itself between the layers of the amnion, while the amnion itself and the ventral laminæ repeat the process observed in the formation of the lamina dorsales.

Everywhere I met with facts of the like apparent significance the coiling up of the intestines would be a simple result of the greater length of the bowel than of the cavity in which they are contained, and answers to a series of such foldings as I first referred to; the convolutions of the cerebrum would necessarily arise from the expansion of its surface within the cranium.1

Instances of this kind, multiplying indefinitely in whatever direction I looked, and becoming more convincing the more carefully they were examined, there was gradually forced upon me the perception that all organic form was determined by simple mechanical conditions. Which conclusion, startling as it appeared on its first enunciation, I had no sooner clearly grasped, than I perceived it to be self-evident. It presented

itself to me thus:

Organic form is the result of motion

Motion takes the direction of least resistance.

1 This has been observed by Mr. Solly in his work upon the Brain.

Therefore organic form is the result of motion in the direction of least resistance.

This is the position which I now propose to illustrate and maintain.

Organic form is the result of motion. By this expression nothing more is meant than that, as we consider form to depend upon the position of the particles of which any body consists, so, in the case of organic bodies, these particles must have assumed their various positions by moving into them. I use it as a postulate in this abstract statement, because it is the simplest formula I can find to express our necessary conception of the facts.

1

That motion takes the direction of least resistance also is an axiom. It is involved in the meaning of the words; for by resistance is meant that which preventing, thereby necessarily directs, the motion.

It is necessary, however, to notice an ambiguity which may here present itself. Motion doubtless takes the direction of least resistance, but every motion must have an original direction, and a momentum which enables it to overcome a greater or less amount of resistance. Do these circumstances detract from or destroy the value of the axiom?

Certainly they do not practically. Mechanics, as an art, reposes on it, and with none the less certainty or success because these conditions have to be remembered.

Nor does the axiom appear to me to be even theoretically defective. It is true every motion must be in a certain direction, but this direction must have been assumed under the operation of the same laws as determine its subsequent course. We here, as in every case, strike upon a chain which has to the human intellect no beginning. Whatever we may suppose concerning the primary origination of motion, of every motion that we

can perceive or conceive we must say that it is such as it is because motion takes the direction of least resistance. And the fact that impulse or momentum overcomes resistance only reminds us that we are apt to use the word resistance in too limited a sense. For what is it that resists motion but force? and what is force but that which, if unresisted, produces motion? It is therefore motion, or the cause of it, that is the true resistance to motion. Thus we of course include the momentum of the moving body among the resistances to be considered, and the axiom assumes the utmost logical completeness. An opposing resistance deflects or changes motion, or is overcome by it, according to whether it be greater or less than the resistance to such change or deflection presented by the momentum. For the momentum clearly becomes a resistance in relation to such change or deflexion. If it were not so, indeed, the axiom itself would be unmeaning.

These few remarks may sufficiently guard against a misconception of the general statement which I have introduced thus broadly. Fortunately there is the less need to dwell upon such speculative views, because the position to be established is a matter of fact and demonstration.

It is remarkable that, in the various hypotheses which have been framed to account for the forms of organic bodies, no attention has been paid to the fact that they are formed, as it were, under pressure; that the process of expansion in which growth consists takes place under conditions which limit it in definite ways. It must surely have been from overlooking this circumstance that a mode of speaking has established itself among us, as if there were in the organic tissue a power of forming itself into peculiar shapes; as if masses of cells, by some power

of their own, could mould themselves into complicated structures. How strangely all such modes of speaking (howsoever they may be disguised, or whatever specious terms may be called in to conceal their nakedness) are at variance with all the principles which are held in regard by us who use them, whether as students of Nature or as professing to recognise a higher agency than Nature's, it is needless to point out. It suffices to show a way of escape from them. For it is certain that such assumptions would never have been tolerated either in our words or in our thoughts, if we had not been driven to them by our inability to refer the phenomena to demonstrable or intelligible causes, such as science, properly so called, concerns herself with.

I say, therefore, that a release from the imagined necessity of assuming such inherent virtues in organic bodies is afforded by two simple facts-1st, that the increase in bulk of developing structures takes place under resistance; and 2dly, that we can, in very many cases, trace the forms assumed by organic bodies, or parts of them, to the operation of the ordinary mechanical laws taken in connection with other conditions known to exist.

In

In some instances it has been found impossible to ignore this relation of morphological changes to mechanical conditions, in spite of opposing hypotheses. the case of the ovum, for example, the cells of the germinal membrane are said to become apparently hexagonal by mutual pressure, arising from the increase of the mass. Doubtless this is natural and true; we could not force ourselves to attribute this change of figure to any other cause. But if this be so, does not the folding of the membrane into laminæ stand in an equally simple relation to pressure from increase of mass? And if some laminæ, why not all? The admission of mechanical

conditions as normal agents in morphological change, in any one instance, involves the necessity of taking them. into consideration in all, and of admitting no other agents except in cases in which these are demonstrably insufficient. How entirely this principle has been neglected it is curious to reflect, nor perhaps does the history of the human intellect furnish a more striking example of the power of an hypothesis to enslave thought and deaden. observation. So intent have we been on pursuing the specific vital tendencies, or the final causes manifested in the uses of the parts, that it would appear as if we had entirely forgotten that living matter is matter after all. The tail (of the cercaria), which was previously employed for locomotion, is now useless, and falls off !"1

Perhaps nothing has contributed so much to divert attention from real to hypothetical causes of morphological change, as the fact that structures entirely alike to the eye, and under analogous external conditions, undergo very different changes. This is especially the case in the development of the ovum, which being alike in almost all animals, so far as we can observe, is yet the parent of the boundless diversity of form that animated nature exhibits. No theory has seemed capable of accounting for this fact but that of a peculiar power

1 Agassiz and Gould's Comp. Phys., p. 343. Bacon's warning has not yet lost its bearing: "To say that the hairs of the eyelids are for a quickset and fence about the sight; or that the firmness of the skins and hides of living creatures is to defend them from the extremities of heat or cold; or that the bones are for the columns or beams, whereupon the frame of the bodies of living creatures is built; or that the leaves of the trees are for protecting of the fruit; or that the clouds are for watering of the earth; or that the solidness of the earth is for the station and mansion of living creatures, and the like, is well inquired and collected in metaphysic; but in physic they are impertinent. Nay, they are indeed but remoras and hindrances to stay and slug the ship from further sailing, and have brought this to pass, that the search of the physical causes hath been neglected and passed in silence.”Advancement of Learning, Book ii.