The Evolution Theory, Vol. 2 of 2

by August Weismann

The same is true of all the unicellular organisms, even of those which are much more simple in structure than the Infusorians, whose differentiation into cortical and medullary substances, oral and anal openings, complex arrangements of cilia and much else, betokens a high degree of differentiation in the cell. But even the Amœba is only apparently simple, for otherwise it could not send out processes and retract them again, creep in a particular direction, encyst itself, and so on, for all this presupposes a differentiation of its particles in different directions, and a definite arrangement of them; and there is in addition the marvellous dividing-apparatus of the nucleus which is not wanting even in the Amœba. All this again points to a historic evolution, a gradual acquiring and an orderly arrangement of differentiations, and such an organism cannot have arisen suddenly like a crystal or a chemical combination.

Thus we are driven back to the lowest known organisms, and the question now before us is whether these smallest living organisms, which are only visible under the highest powers of the microscope, may be referred to spontaneous generation. But here too the answer is, No; for although there is no nucleus to be found, and no substance which we can affirm with any certainty to be composed of primary constituents or idioplasm, we do find distinct traces of a previous history, and not the absolutely simple structure of homogeneous living particles, unarranged in any orderly way, which is all that could be derived from spontaneous generation. It has been shown quite recently that the typhus bacillus possesses an extremely delicate much-branched tuft of flagella, which gives it a tremulous motion, and in the cholera bacillus cortical and medullary substances can be distinguished. Thus even here there is differentiation according to the principle of division of labour, and how numerous must be the minute vital particles of which a substance consists when it can form such fine threads as the flagella just mentioned! Nägeli, who elaborated an analogous train of thought in regard to spontaneous generation, calculated the number of these smallest vital particles (his 'micellæ') which must be contained in a 'moneron' of 0.6 mm. diameter, if we take its dry substance at 10 per cent., and he arrived at the amazing figure of 100 billions of vital particles. Even if we suppose the diameter of such an organism to be 0.0006 mm., it would still be composed, according to this calculation, of a million of these vital particles.

We have reached, in the course of these lectures, the conviction that minute living units form the basis of all organisms, namely, our 'life-bearers' or 'biophors.' These must be present in countless multitudes, and in a great number of varieties in the different forms of life, but all agree in this, that they are simple, that is, they are not composed in their turn of living particles, but only of molecules, whose chemical constitution, combination, and arrangement are such as to give rise to the phenomena of life. But they may vary, and on this power depends the possibility of their differentiation, which has taken place in more and more diverse ways in the course of phylogeny. They, too, arise in the existing organism, like all vital units, only by multiplication of the biophors already present, but they do not necessarily presuppose a historic origin; it is conceivable of them, at least as far as their first and simplest forms are concerned, that they may have arisen some time or other through spontaneous generation. In regard to them alone is the possibility of origin through purely chemico-physical causes, without the co-operation of life already existing, admissible. It is only in regard to them that spontaneous generation is not inconceivable.

We must, therefore, assume that, at some time or other in the history of the earth, the conditions necessary to the development of these invisible little living particles must have existed, and that the whole subsequent development of the organic world must have depended upon an aggregation of these biophors into larger complexes, and upon their differentiation within these complexes.

We shall never be able, then, directly to observe spontaneous generation, for the simple reason that the smallest and lowest living particles which could arise through it, the Biophoridæ, are so extremely far below the limits of visibility, that there is no hope of our ever being able to perceive them, even if we should succeed in producing them by spontaneous generation.

I do not propose to discuss the chemical problem raised by the possible occurrence of spontaneous generation. We have already seen that dead protoplasm, in addition to water, salts, phosphorus, sulphur, and some other elements, chiefly and invariably contains albumen; an albuminoid substance must, therefore, have arisen from inorganic combinations. No one will maintain that this is impossible, for we continually see albuminoid substances produced in plants from inorganic substances, compounds of carbon and nitrogen; but under what conditions this would be possible in free nature, that is, outside of organisms, cannot as yet be determined. Possibly we may some time succeed in procuring albumen from inorganic substances in the laboratory, and if that happens the theory of spontaneous generation will rest upon a firmer basis, but it will not have been experimentally proved even then. For while dead albumen is certainly nearly allied to living matter, it is precisely life that it lacks, and as yet we do not know what kinds of chemical difference prevail between the dead proteid and the living; indeed we must honestly confess that it is a mere assumption when we take for granted that there are only chemico-physical differences between the two. It cannot be proved, in the meantime, that there is not another unknown power in the living protoplasm, a 'vitalistic principle,' a 'life-force,' on the activity of which these specific phenomena of life, and particularly the continually repeated alternation of disruption and reconstruction of the living substance, dissimilation and assimilation, growth and multiplication, depend. It is just as difficult to prove the converse, that it is impossible that chemico-physical forces alone should have called forth life in a chemical substance of very special composition. Although no one has ever succeeded, in spite of many attempts, in thinking out a combination of chemical substances which—as this wonderful living substance does—on the one hand undergoes combustion with oxygen and, on the other hand, renews itself again with 'nutritive' material, yet we cannot infer from this the impossibility of a purely chemico-physical basis of life, but must rather hold fast to it until it is shown that it is not sufficient to explain the facts, thus following the fundamental rule that natural science must not assume unknown forces until the known ones are proved insufficient. If we were to do otherwise we should have to renounce all hope of ever penetrating deeper into the phenomena. And we have no need to do this, for in a general way we can quite well believe that an organic substance of exactly proportioned composition exists, in which the fundamental phenomena of all life—combustion with simultaneous renewal—must take place under certain conditions by virtue of its composition.

How, and under what external conditions, such a substance first arose upon the earth, from and of what materials it was formed, cannot be answered with any certainty in the meantime. Who knows whether the fantastic ideas of Empedocles in an altered form would not be justified here? I mean that, at the time of the first origin of life, the conditions necessary for many kinds of complex chemical combinations may have been present simultaneously on the earth, and that, out of a manifold variety of such substances, only those survived which possessed that marvellous composition which conditioned their continual combustion, but also their ceaseless reconstruction by multiplication. According to Empedocles, there arose from chaos only parts of animals—heads without bodies, arms without trunks, eyes without faces, and so on—and these whirled about in wild confusion and flew together as chance directed them. But those only survived which had united rightly with others so as to form a whole, capable of life. Translated into the language of our time, that would mean what I have just said—that, of a large number of organic combinations which arose, only a few, perhaps one, would possess the marvellously adjusted composition which resulted in life, and with it self-maintenance and multiplication; and that would be the first instance of selection!

But let us leave these imaginings, and wait to see whether the chemists will not possibly be able to furnish us with a starting-point for a more concrete picture of the first origin of life. In the meantime, we must confess that we find ourselves confronted with deep darkness.

The question as to the 'Where' of spontaneous generation must also be left without any definite answer. Some have supposed that life began in the depths of the sea, others on the shore, and others in the air. But who is to divine this, when we cannot even name theoretically the conditions and the materials out of which albuminoid-like substances might be built up in the laboratory? Nägeli's hypothesis still seems to me to have the greatest probability. According to his theory, the first living particles originated not in a free mass of water, but in the reticulated superficial layer of a fine porous substance (clay or sand), where the molecular forces of solid, fluid, and gaseous bodies were able to co-operate.

Only so much is certain, that wherever life may first have arisen upon this earth, it can have done so only in the form of the very simple and very minute vital units, which even now we only infer to be parts of the living body, but which must first have arisen as independent organisms, the 'Biophoridæ.' As these, according to our theory, possessed the character of life, they must have possessed above all the capacity of assimilating in the sense in which the plants assimilate, that is, of renewing their bodily substance continually from inorganic substances, of growing, and of reproducing. They need not on that account have possessed the chemical constitution of chlorophyll, although the capacity of assimilation in green plants depends upon this substance, for we know colourless fungi, which, notwithstanding the absence of chlorophyll, are able to build up the substance of their body from compounds of carbon and nitrogen.