separated by no wide impassable barrier from other kinds of matter. It is acted upon by, and reacts on energy; but this will only distinguish it from other substances in degree, as they also are so acted upon. It increases by assimilation of suitable material; but such increase is not entirely peculiar to it. Lastly, the difference between dead and living organic matter is probably chemical, or due to a new arrangement of atoms. Hence there is no reason to believe that protoplasm cannot have been evolved from inorganic material; and, as we have already seen, analogy and the belief in the uniformity of Nature's laws lead us to the conclusion that it probably has been so evolved.

THE tendency of protoplasm to acquire structure has already been referred to; but the acquirement of structure is not necessary for the manifestation of vitality. The simplest living forms exhibit no structure; and cells possessing the highest physiological endowments scarcely differ in this respect from those which perform no active function; indeed the latter class are often far more complex than the former. On the other hand, structure may be acquired by inorganic bodies under the influence of energy. The beautiful structure of melting ice, the crystallization of the axles of railway-engine-wheels under repeated mechanical shocks, and that of selenium under the influence of light, the fibrilla

tion of certain minerals (as amianthus and asbestos) as well as of the blood-clot, are examples of structure arising in dead material.

Herbert Spencer insists strongly that the tendency of all homogeneous substances to become heterogeneous, from the unequal action of various forces upon them, is a powerful cause of the origin of structure; and there can be no doubt that the various modifications of this law, which he calls "the instability of the homogeneous,' are extremely potent in their action. Just as a portion of wax cools unequally, so that its periphery becomes solid before its centre, a mass of protoplasm may acquire an external envelope harder and more dense than its central portion. This is exactly the condition of the little Amoeba of our ditches; and thus we have the first indication of the manner in which a cellwall may originate.

As we have already seen, protoplasm has the power of oxidizing its food-material, and of setting free potential energy from carbon compounds. The Amoeba or Proteus feeds upon such material by imbedding it in its interior; and during its oxidation potential energy is rendered sensible. This energy, by acting on the particles of protoplasm, gives rise to molecular movements. No structure is originated by the internal forces thus set free; but

*First Principles, 2nd edit. p. 401.

movements of the mass result. The external denser body-wall, which may fairly be looked upon as the result of external influences acting upon the mass, is sufficiently soft to move with the interior; and pseudopodia are pushed out and drawn in again around its periphery. If a viscid drop of caramel or burnt sugar be allowed to fall into water, the action of the water upon it forms an external envelope. Finger-like processes will then be protruded, probably from the imbibition of water by the mass; these bear a striking resemblance to the pseudopodia of an Amoeba; and although their origin is undoubtedly due to a totally distinct cause, they serve to show that a force acting throughout the mass, as one may readily conceive the forces of an Amoeba act, may give rise to extremely unsymmetrical processes, quite like pseudopodia.

It appears probable (although at present we have not sufficient evidence on this subject) that, if the energy which gives rise to slow irregular movements in the Amoeba were pent up by the more complete hardening of the body-wall, it would either give rise to molecular rearrangements within (as it actually does in the encysted Amoeba), or it would be collected in one point, thus giving rise to more energetic movements of one part of the surface. It appears probable that the production of flagella and cilia may be thus determined.

Numerous Foraminifera produce flask-like shells open at that part which corresponds to the mouth of the flask; the pseudopodia are then formed at one part of the surface of the animal only-that corresponding to the opening. This arrangement is probably not entirely due to the formation of a shell around the body of the animal, because a form of rhizopod, Lieberkuhnia, which has no shell, puts forth one large manybranched pseudopod, the offshoots from which form a complex reticulation around its body *. In this case the material of the central nucleus and that of its reticulate pseudopodial system are constantly changing places, a regular circulation passing from one to the other. We may rest assured that the protrusion of a single branching stolon is the result of the influence of external or internal physical conditions. The influence of currents, temperature, and light may determine the point of the viscid nucleus which is prolonged, perhaps by a directive influence over the currents of the protoplasmic matter, or perhaps by determining a polarity of its atoms.

Such a creature as this, having once acquired a body-wall, would necessarily protrude its pseudopodia from a single point; and these, it may be readily conceived, would also become altered by

* Carpenter, Foraminifera' (Ray Society).