peculiar cells that previously passed from the apex of the embryo sac to that of the mammillary protuberance of the nucleus."1

Just such, again, is the germination of the seed: the decomposition of the albumen produces at once vital action and expansion, and growth takes place first in the radicle, then in the plumule, these being the directions of least resistance.

May not the curious fungus which forms in certain caterpillars be classed with these? First occupying the body of the animal, it finds its way out as it increases always at the junction of the head with the body, the direction of least resistance.

My conception of the nature of these changes is illustrated by the fact that separated portions of cactus will grow and increase in size while they gain no increase of weight. The starting-point here I take to be a decomposition analogous to that which takes place in a germinating seed.

Does not the power of repair resolve itself into an instance of growth in the direction of least resistance? Is not a wound an axil? and the granulations which form in it, or the new member which grows in the place of a lost one, do not they correspond to the buds which form in axils in the growth of plants or the development of the embryo? The solution of continuity removes the resistance of the external investiture. Is there, therefore, any basis for the supposition of a special power by which a living body can recover itself from accident or injury? The law of its formation involves its repair. So if some leaves be incised, buds spring up from the cut surface; the hydra gemmates from a wound. These are artificial axils. I do not mean to imply that no other circum1 Op. cit., p. 896.

stances are to be regarded in relation to the reparative process. Irritation, of whatever kind, produces special modifications of the vital action, but I suggest that the general fact of the repair of wounds is an illustration of growth taking the direction of least resistance. The new material is deposited where the resistance to expansion is removed; is it not deposited there rather than in other portions of the body, because the resistance at that point is least? We know that repair is effected at the expense of the general nutrition, and we know, too, the effect of scabbing, or pressure otherwise applied, in limiting the process of granulation. Perhaps I may state the case thus if growth take the direction of least resistance (other circumstances being the same), then it is certain that wounds must be repaired.

In truth an entirely new conception of homology arises out of the recognition of a LAW OF FORM; a parallelism of various organs, according to the dynamical conditions exhibited in their morphology, which embraces all parts of all bodies, and extends itself through the whole of organic nature. On this point I will confine myself to one suggestion. Observe the form of the intestinal canal (I speak of the mammalia), straight for a short distance from its orifice, then convoluted, then terminating in an expanded portion, the stomach. Compare this now with the form of the tubuli in the kidney-a straight portion, a convoluted portion, an expansion, the Malpighian corpuscle. Again, take the nervous tubules in the brain; there is a straight portion, the white substance; is not the grey matter a convoluted cortical portion (like that of the kidney), and are not the cells intermingled with the convoluted fibres expansions, not without a formal resemblance to the Malpighian bodies? I do not wish to erect any strainalogy; perhaps this is a dane fancy alto

gether, but to my mind there is indicated by this similarity of form a similarity of mechanical conditions not. without its interest.

Development, then, is due to increase under limit; it is determined by resistance. Is it not self-evident? Conceive an ovum germinating with all other circumstances unaltered, but with no external limitations, no membranes, no uterus, nothing to check expansion in any form. Could anything else result but a shapeless multitude of cells? But if it be so, let us fairly face the position that we take. The mechanical limitations must act mechanically, and form be the result of mechanical conditions. Consider how every organism intended to develop is subject to external resistance, the tough capsule of seeds, the shell of eggs, the womb of the vivipara. Are not the marsupials and monotrems which escape so early from the uterus less developed than animals whose gestation is more prolonged? Think of the firm sheath of every muscle, the capsule of every viscus, the bony case of the nervous system; remember how every free surface in the body is covered with cells, and with cells alone. If the membranes of the brain be divided, a cellular fungoid growth protrudes; is it not that the maintenance of the organisation of the brain demands. the resistance of its coverings?

Using the term "uterus," therefore, to denote a definite external resistance to extension, is it not an axiom that everything is DEVELOPED in a uterus?

It should be remarked here that the forms of parts of animals are often greatly altered, after their first development, by growth under conditions different from those in which the development takes place. Growth modifies the form which development primarily determines; the body moulded within the uterus, expands freely there

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after without external resistance. Hence the result is changed, but not the law. A single instance will make clear my meaning: in the early bud the anther constitutes the entire length of the stamen; as the flower expands the stalk has room to grow.

There is one other class of cases to which it is necessary to refer; those, namely, in which the form immediately results not from growth or expansion, but from wasting; an extreme instance of which is presented in the cellular formations constituting the pith of plants. Indications of such relative decay are of great frequency in the animal body, especially in the higher grades of development, but this slight mention may suffice for them here. It is clear that the law of least resistance, which means no more than that mechanical conditions determine mechanical results, applies equally to them. The wrinkling up of the lining of the corpus luteum, partly we may be sure from the contraction of the capsule, is a marked example.

If it should be remarked that there exist in developing structures certain definite modes or operations of force, such as attractions or repulsions in particular directions, which serve to determine the form assumed, apart from any influence of the visible mechanical conditions, this is willingly admitted to be true. The morphological law suggested does not contravene, but rests upon, these phenomena. They may be regarded in two ways; either as constituting part of the molecular process in which nutrition consists, as instances of those local manifestations of growth before referred to, and which are presupposed as the foundation on which the law is based; or perhaps more properly they may be themselves. considered as coming within its scope. In so far as these changes consist in the motion of particles, the law

of least resistance may be asserted of them, or at least cannot be denied. Such molecular changes, indeed, form no portion of the evidence on which the proposition can be based, inasmuch as the nature of the process and all its conditions are as yet beyond our investigation. But that in so far as they consist in motion they conform to the nature of motion, we may be quite sure. The structure of the germ must be such as to determine the operation of whatever chemical or other forces come into play within it, to produce motion in these particular directions.

This, then, is my argument. The illustrations I have adduced may be insufficient, or unsatisfactory, or false, or misconceived, but no defect of this kind in the proof can affect the proposition; for it rests upon necessary laws of thought. Physical morphology is like an applied geometry; if I have failed in the application, others will certainly succeed.1

Resistance to motion is of necessity an opposing force; force and resistance are indeed interchangeable terms, two aspects of the same thing, as when the two hands are pressed together, each mutually resists the force applied by the other. Viewed in relation to this law of least resistance, therefore, the idea of organisation is beautiful. It is the result of motion in the direction of least opposing force. Certainly: how should it be anything else? Is not organisation a perfect mutual adaptation and exact conformity to each other of all the parts, even to the minutest details, an absolute rightness and order? And how should this be attained except through

1 It may be urged that in magnetism and other forces, and in human actions, we have instances of motion to which this conception of least resistance is not applicable. It would be too great a licence to enter on a discussion of these matters under cover of an inquiry into organic form; but it appears to me that, in so far as they come within the domain of the physical, the conception of least resistance as determining the form and direction of the action is neither inapplicable nor infertile in results.