for the present all consideration of the tendency to assume the inherited type.
Note first what appear to be exceptions, as the Amœba. In this creature and its allies, the substance of the jelly-like body remains throughout life unorganized – undergoes no permanent differentiations. But this fact, which seems directly opposed to our inference, is really one of the most significant evidences of its truth. For what is the peculiarity of the Rhizopods, exemplified by the Amœba? They undergo perpetual and irregular changes of shape – they show no persistent relations of parts. What lately formed a portion of the interior is now protruded, and, as a temporary limb, is attached to some object it happens to touch. What is now a part of the surface will presently be drawn, along with the atom of nutriment sticking to it, into the centre of the mass. Thus there is an unceasing interchange of places; and the relations of inner and outer have no settled existence. But by the hypothesis, it is only in virtue of their unlike positions with respect to modifying forces, that the originally-like units of a living mass become unlike. We must not therefore expect any established differentiation of parts in creatures which exhibit no established differences of position in their parts.
This negative evidence is borne out by abundant positive evidence. When we turn from these ever-changing specks of living jelly to organisms having unchanging distributions of substance, we find differences of tissue corresponding to differences of relative position. In all the higher Protozoa, as also in the Protophyta, we meet with a fundamental differentiation into cell-membrane and cell-contents, answering to that fundamental contrast of conditions implied by the words outside and inside. And on passing from what are roughly classed as unicellular organisms to the lowest of those which consist of aggregated cells, we equally observe the connexion between structural differences and differences of circumstance. In the sponge, permeated throughout by currents of sea-water, the absence of definite organization corresponds with the absence of definite unlikeness of conditions. In the Thalassicolla of Professor Huxley – a transparent, colourless body, found floating passively at the surface of the sea, and consisting essentially of "a mass of cells united by jelly" – there is displayed a rude structure obviously subordinated to the primary relations of centre and surface: in all of its many and important varieties, the parts exhibit a more or less concentric arrangement.
After this primary modification, by which the outer tissues are differentiated from the inner, the next in order of constancy and importance is that by which some part of the outer tissues is differentiated from the rest; and this corresponds with the almost universal fact that some part of the outer tissues is more directly exposed to certain environing influences than the rest. Here, as before, the apparent exceptions are extremely significant. Some of the lowest vegetable organisms, as the Hematococci and Protococci, evenly imbedded in a mass of mucus, or dispersed through the Arctic snow, display no differentiations of surface: the several parts of the surface being subjected to no definite contrasts of conditions. The Thalassicolla above mentioned, unfixed, and rolled about by the waves, presents all its sides successively to the same agencies; and all its sides are alike. A ciliated sphere like the Volvox has no parts of its periphery unlike other parts; and it is not to be expected that it should have; seeing that as it revolves in all directions, it does not, in traversing the water, permanently expose any part to special conditions. But when we come to creatures that are either fixed, or while moving, severally preserve a definite attitude, we no longer find uniformity of surface. The gemmule of a Zoophyte, which during its locomotive stage is distinguishable only into outer and inner tissues, no sooner takes root than its upper end begins to assume a different structure from its lower. The free-swimming embryo of an aquatic annelid, being ovate and not ciliated all over, moves with one end foremost; and its differentiations proceed in conformity with this contrast of circumstances.
The principle thus displayed in the humbler forms of life, is traceable during the development of the higher; though being here soon masked by the assumption of the hereditary type, it cannot be traced far. Thus the "mulberry-mass" into which a fertilized ovum of a vertebrate animal first resolves itself, soon begins to exhibit a difference between the outer and inner parts answering to the difference of circumstances. The peripheral cells, after reaching a more complete development than the central ones, coalesce into a membrane enclosing the rest; and then the cells lying next to these outer ones become aggregated with them, and increase the thickness of the germinal membrane, while the central cells liquefy. Again, one part of the germinal membrane presently becomes distinguishable as the germinal spot; and without asserting that the cause of this is to be found in the unlike relations which the respective parts of the germinal membrane bear to environing influences, it is clear that we have in these unlike relations an element of disturbance tending to destroy the original homogeneity of the germinal membrane. Further, the germinal membrane by and by divides into two layers, internal and external; the one in contact with the liquefied interior part or yelk, the other exposed to the surrounding fluids: this contrast of circumstances being in obvious correspondence with the contrast of structures which follows it. Once more, the subsequent appearance of the vascular layer between these mucous and serous layers, as they have been named, admits of a like interpretation. And in this and the various complications which now begin to show themselves, we may see coming into play that general law of the multiplication of effects flowing from one cause, to which the increase of heterogeneity was elsewhere ascribed.9
Confining our remarks, as we do, to the most general facts of development, we think that some light is thus thrown on them. That the unstable equilibrium of a homogeneous germ must be destroyed by the unlike exposure of its several units to surrounding influences, is an a priori conclusion. And it seems also to be an a priori conclusion, that the several units thus differently acted upon, must either be decomposed, or must undergo such modifications of nature as may enable them to live in the respective circumstances they are thrown into: in other words —they must either die or become adapted to their conditions. Indeed, we might infer as much without going through the foregoing train of reasoning. The superficial organic units (be they the outer cells of a "mulberry-mass," or be they the outer molecules of an individual cell) must assume the function which their position necessitates; and assuming this function, must acquire such character as performance of it involves. The layer of organic units lying in contact with the yelk must be those through which the yelk is absorbed; and so must be adapted to the absorbent office. On this condition only does the process of organization appear possible. We might almost say that just as some race of animals, which multiplies and spreads into divers regions of the earth, becomes differentiated into several races through the adaptation of each to its conditions of life; so, the originally homogeneous population of cells arising in a fertilized germ-cell, becomes divided into several populations of cells that grow unlike in virtue of the unlikeness of their circumstances.
Moreover, it is to be remarked in further proof of our position, that it finds its clearest and most abundant illustrations where the conditions of the case are the simplest and most general – where the phenomena are the least involved: we mean in the production of individual cells. The structures which presently arise round nuclei in a blastema, and which have in some way been determined by those nuclei as centres of influence, evidently conform to the law; for the parts of the blastema in contact with the nuclei are differently conditioned from the parts not in contact with them. Again, the formation of a membrane round each of the masses of granules into which the endochrome of an alga-cell breaks up, is an instance of analogous kind. And should the recently-asserted fact that cells may arise round vacuoles in a mass of organizable substance, be confirmed, another good example will be furnished; for such portions of substance as bound these vacant spaces are subject to influences unlike those to which other portions of the substance are subject. If then we can most clearly trace this law of modification in these primordial processes, as well as in those more complex but analogous ones exhibited in the early changes of an ovum, we have strong reason for thinking that the law is fundamental.
But, as already more than once hinted, this principle, understood in the simple form here presented, supplies no key to the detailed phenomena of organic development. It fails entirely to explain generic and specific peculiarities; and leaves us equally in the
See Essay on "Progress: its Law and Cause."
