Part 29

In many-celled organisms, however, we must admit that there is an essential difference between personal and histonal selection, inasmuch as the latter can give rise to adaptive structural modifications corresponding to the needs of the tissue at the moment, but not to permanent and cumulative changes in the individual elements of the tissue. If a broken bone heals crookedly, the spongy substance within the healed portion does not remain as it was before, for the pillars and arches, which now no longer run in the direction best suited to their function, break up, and a new system of arches is formed, not in line with the earlier one, but adapted to the new conditions of pressure. This is certainly an adaptation through selection, but the elements, that is the cells which form the bone substance in response to strain and pressure, or those which in response to the stimulus of the blood flowing into the spaces form the blood-vessels, or those which being quite freed from one-sided pressure develop into connective tissue, must be presupposed. These kinds of cells must be virtually implied in the germ-rudiment; they are themselves adaptations of the organism, and can therefore only be referred to _personal selection_. And this is true of all adaptations of the _elements_ of multicellular organisms, and thus of the _cells_. Their adaptation according to the principle of division of labour, their differentiation into muscle, nerve, and gland cells can only be referred to natural selection in the Darwin-Wallace sense, and cannot depend upon histonal selection. In the spongy substance of the bone a better bone-cell does not struggle with an inferior one and leave behind it by its survival a host of descendants which are, if possible, better than itself; the struggle for existence and for descendants, in this case, is between two kinds of cell which were different from the beginning, and of which one has the advantage at one spot, another at another. The case may be compared to that of a flock of nearly allied species of bird, of which one species thrives best in the plains, another among the hills, and a third among the mountain forests, all mingled together in a vast new territory to which they had migrated, and in which all three kinds of conditions were represented. A struggle would arise among the different species, in which in every case the particular species would be victorious which was best adapted to the local conditions. But each would thrive best in the region in which it was superior to the others, and very soon the three species would be distributed as they were in the land from which they came--in the plains, the high lands, and the mountain forests. This would be the result of a struggle between the three species, _not between individuals within each species_, and it could not therefore bring about an improvement of a single species, but only the local prevalence of one or another. The characters which made one species adapted for the plain, another for the mountain forest were _already there_; they can only be referred to personal selection, which brought about the adaptation of their ancestors in the course of ages to the conditions of their life. Something similar is true of the adaptations of the tissues; the differentiation of the individual kinds of cells is an ancient inheritance, and depends upon personal selection, but their distribution and arrangement into specially adapted tissues, so far as there is any plasticity at all, depends upon histonal selection. Obviously, however, only as far as the tissue is plastic, that is, with the power of adjusting itself to particular local conditions. Only adaptations of this kind can be referred to histonal selection; the ground-plan, even of the most complicated tissue, such as the large glands of mammals, the kidneys, the liver, and so on, must have been implicit in the germ, and must therefore be referred to personal selection. A precise limitation of the respective spheres of action of personal selection and histonal selection is not possible as yet, since hardly any investigations on the subject are available.

Roux undoubtedly over-estimated the influence of his 'struggle of parts' when he believed that the most delicate adaptations of the different kinds of cells depended on it. I admit that, for a considerable time, I made the same mistake, until it became clear to me, as it did first in regard to the sex-cells, that this is not, and cannot be the case. How, for instance, could the diverse and minutely detailed adaptations of the sex-cells--which we are to discuss in a subsequent lecture--have arisen in this way? As far as the individual sperm-cell is concerned, it is a matter of indifference whether its head is a little thinner or thicker, its point a little sharper or blunter, its tail a little stronger or weaker. This does not decide whether the cell is to thrive better, or to occur in greater numbers than some other variety. But it does decide whether it is to be able to penetrate through the minute micropyle, or through the firm egg-envelope, into the egg, there to effect fertilization. An individual with less well formed sperm-cells will be able to fertilize fewer eggs, and therefore to leave fewer descendants which might inherit its tendency to produce inferior sperm-cells, and conversely. Thus it is not the sperm-cells of any one individual which are selected according to their fitness, it is the individuals themselves which compete with one another in the production of germ-cells which shall fertilize best, that is, most certainly. The struggle is thus not intercellular, but a struggle between persons.

The same is true of all cells differentiated for particular functions; every new kind of glandular, muscular, or nerve cell, such as have arisen a thousandfold in the course of phylogeny, can only have resulted from a struggle between individuals which turned on the possession of the best cells of a particular kind, _not from a struggle between the cells themselves_, since these would gain no advantage from serving the organism, as a whole, better than others of their kind. In regard to the sex-cells we might admit, in addition to personal selection, the possibility of an internal struggle between the sperm-cells or egg-cells of the same individual, inasmuch as each of these cells is the primordium of a new individual, and as those better adapted for reproduction might transmit their better quality to these new individuals. I will not here enter into my reasons for regarding this idea as erroneous, for in any case this interpretation would not apply to any other kind of cells. If, for instance, it were a question of the transformation of an ordinary mucus or salivary gland into a poison gland, it would not matter in the least to the individual cell whether it yielded a harmless or a poisonous secretion; but individuals with many poisonous cells would have an advantage in the struggle for existence.

I agree so far with Plate when he refers the differentiation of the tissues entirely to personal selection, but not in his further conclusion that histonal selection does not exist. The ground-plan of the architectural structure of the organ depends upon personal selection, but the realization of the plan in particular cases is not predetermined down to the minutest details, but is regulated by histonal selection, and is thus to a certain extent an adaptation to local conditions of stimulus. The direction, strength, and size of every single bone lamella is not predetermined from the germ, but only the occurrence and nature of bone-cells and bone lamellæ in general. The direction and the strength which these bone lamellæ may assume depends on the local conditions of strain and pressure which affect the cell-mass, as is shown very clearly by the spongiosa of an obliquely healed bone, which we have already described.

But let us now turn to the question which is here most important for us: _whether functional adaptations can be transmitted_. We must admit that the insight we have so far gained into the causes of these adaptations does not make it much easier to answer the question. Histonal selection is a purely _local_ process of adaptation to the conditions of stimuli prevailing at the moment, and no one will be likely to suppose that the distorted position of the spongiosa of a badly healed fracture could reappear in the straight bone of a descendant; this would be quite contrary to the principle, for the crooked lamellæ would in that case no longer be the best adapted. Even the question _whether the strengthening of a muscle through use can be transmitted_ cannot be answered in the light of the knowledge we have hitherto gained. The 'trophic effect of the functional stimulus' is brought into activity through entirely local influences, through which only the parts most strongly affected by the stimulus can be caused to vary. Thus the problem remains unaltered, How can purely local changes, not based in the germ, but called forth by the chances of life, be transmitted to descendants?

If all species, even in the highest groups, reproduced by dividing into two, we might imagine that a direct transmission of the changes acquired in the course of the individual life through use or disuse took place, though this would presuppose a much more complicated mechanism than is apparent at first sight. But it is well known that multiplication by fission is for the most part restricted to simple organisms, and that the great majority of modern plants and animals reproduce by means of germ-cells, which develop within the organism in regions often very remote from the parts, the results of the exercise of which are said to be transmitted. Moreover, the germ-cells are of very simple structure, at least as far as our eyes can discern; for we see in a germ-cell neither muscles nor bones nor ligaments, glands nor nerves, but only a cell-body consisting of that semifluid living matter to which the general name of protoplasm has been given, and of a nucleus, in regard to which we cannot say that it differs in any essential or definite way from the nucleus of any other cell. How then could the changes which take place in a muscle through exercise, or in the degeneration of a joint in consequence of disuse, communicate themselves to a germ-cell lying inside the body, and do so in such a fashion that this germ-cell is able, when it grows into a new organism, to produce of itself, in the relevant muscle and joint, a change the same as that which had arisen in the parent through use and disuse? That is the question which forced itself upon me very early, and in following it up I have been led to an absolute denial of the transmission of this kind of 'acquired characters.'

In order to explain how I reached this result, and what it is based upon, it is indispensable that we should first make ourselves acquainted with the phenomena of heredity in general, and the inseparably associated phenomena of reproduction, so that we may form some sort of theoretic conception of the process of inheritance--a picture, necessarily provisional and imperfect, of the mechanism which enables the germ-cell to reproduce the whole organism, and not merely, like other cells, others like itself. We are thus led to an investigation of reproduction and heredity, at the conclusion of which we shall feel justified in returning to the question of the inheritance of acquired characters, in order to give a verdict as to the retention or dismissal of the Lamarckian principle.

LECTURE XIII

REPRODUCTION IN UNICELLULAR ORGANISMS

Reproduction by division--In Amœbæ--In Infusorians--Divisions following one another in immediate succession--Formation of germ-cells in the Metazoa--Contrast between germ-cells and body-cells--Potential immortality of unicellular organisms--Beginning of natural death--Budding and division in the Metazoa.

WE wish to consider the reproduction of organisms with special reference to the problem of heredity, and it is most instructive to begin with the lowest forms of life--the unicellulars--because their structure, as far as we can see with the instruments at our command, is very simple, and, what is even more important, is relatively homogeneous.

Suppose that there are bacteria-like organisms of quite homogeneous structure, and that these multiply by simply dividing into two, each rod-like creature dividing transversely in the middle of its length, the two halves would represent independent daughter-organisms, whose structure would correspond exactly with that of the mother-organism, could not indeed in any way deviate from it, and consequently would take over all its characters, that is, would inherit them. The size of body is the only feature which is not obviously inherited, but in reality it is potentially heritable, since the structure of the divided portions involves the capacity and the limits of their possible growth. Moreover, the size of body is not invariable in any species; a particular size is only reproduced under similar conditions of development. Inheritance here consists simply in a continuation of the mother-organism into its two daughter-cells.

Even in an Amœba (Fig. 59) we might picture the process of inheritance as equally simple, though in so doing we should probably be making a fallacious inference, for the structure of these lowest unicellular animals probably seems to us simpler and more homogeneous than it really is. Among Infusorians it is quite obvious that inheritance implies more than the mere halving of the mother-animal into the two daughter-cells; something more must be involved. For among these unicellular animals the differentiation of the body is not only great, but it is unsymmetrical. The posterior and the anterior ends are different, and the transverse division of the animal, in which the process of reproduction here consists, does not produce two halves, but two very unequal portions. In the division of _Stentor_, the so-called trumpet-animalcule (Fig. 60), the anterior portion contains the funnel-shaped mouth and gullet with its complicated nutritive apparatus, the circular peristome with its spirally curved rows of composite ciliated plates, the so-called membranellæ, and so forth; the posterior half contains nothing of all this, but possesses the foot of the mother-Stentor with its attaching organ, which the anterior half lacks. But each of the two portions possesses the power of 'regeneration,' that is, it is able to develop anew the missing parts, mouth or foot, and so on. So that here there is no longer merely a simple continuance of the maternal organization in the daughter-animals, there is something new added, something which requires explanation; we are confronted with the first riddle of heredity. Simple growth does not explain the phenomenon, for what has to be added to complete the halved portions has a different structure, a different form, different accessory apparatus from any that the halves themselves possess. It in no way affects this state of matters that in the normal process of division in Infusorians the formation of the new mouth and peristome-region begins before the halves have actually separated, for even if a Stentor be cut in two artificially the cut halves form complete animals. And, indeed, a Stentor may be cut into three or four pieces, and in certain conditions each piece will develop into a complete animal. These pieces must therefore possess something more than the mere power of growth. We shall try later on to discover whether this marvellous invisible transmission of characters, this implication of the whole in each of the parts, can be in any way theoretically expressed and included in our scheme of conceptual formulation.

Now that we have become familiar with these facts it will no longer surprise us to learn that the reproduction of unicellular animals does not always depend on _equal_ division, but that unequal spontaneous divisions are also possible, so that one or several smaller portions of the cell-body, containing a portion of the cell-nucleus, can separate off from the mother-animal. This occurs especially among the suctorial Infusorians or Acinetæ. In relation to the phenomena of inheritance the problem raised by the equal division of the Infusorians repeats itself, and it is in no way affected by the fact that equal division can take place several times, or many times in succession, so that from one animal a large number of small pieces of the same size may be produced in rapid succession. The characteristic marks of the mother-animal are not infrequently lost sight of, wholly or partially, when this occurs, and the divided portions seem to consist of a homogeneous cell-body and nucleus; but they possess the power of regenerating themselves, or of developing, if the expression be preferred, into animals similar to the maternal-organism. Such divided portions might very well be called germs, only it must not be forgotten that the relation of the mother-animal to these germs is a different one from that of a higher animal or plant to its germ-cells; the unicellular animal breaks up by continued division into these 'germs,' while the Metazoon continues its individual existence unimpaired by the production of its germ-cells.

The beginning of a so-called 'spore-formation' is to be found in many Infusorians. Thus the holotrichous species, _Holophrya multifiliis_ (Fig. 61), reproduces by first becoming enclosed in a cyst or capsule, and then dividing many times in rapid succession, so that 2, 4, 8, 16, &c. individuals arise consecutively, and subsequently burst forth from the cyst (Fig. 61, _B_). In the Gregarines and other Sporozoa the period of division lasts much longer, and the encysted animal divides into 128, 256, or even more portions; but in this case also each part or 'spore' receives a piece of the maternal cell-body and cell-nucleus, so that there is no difference in principle between this and the simple division into two exhibited by _Stentor_; as in that case, so here, it is not the fully differentiated structure of the animal which is handed on to the divided parts; it is only the power to redevelop this anew on their own account. Thus here again we are face to face with the fundamental problem of heredity: How is it possible that the power of reproducing the complex whole can be inherent in the simple parts?

In contrast to the unicellular organisms, the great majority of the multicellulars, the Metazoa and Metaphyta, many-celled animals and plants, differ not only in the multitude of their cells, but even more in the manifold differentiation of these cells according to the principle of division of labour, so that the various functions of the animal are not performed by all the cells uniformly, but each function is relegated to a particular set of cells specially organized with reference to it. Thus there is differentiation between motile, nutritive, and reproductive cells, and there may also be glandular, nerve, muscle, and skin cells, and we know how this differentiation into a great number of different kinds of cells with highly specialized functions has arisen, especially among the higher animals, in a multiplicity which cannot easily be overlooked. Thus we find a large number of the most diverse kinds of cells, all of which serve for the maintenance of the body, in contrast to the simply reproductive cells or germ-cells. These alone possess the power of reproducing, under certain conditions, a new individual of the same species. We can contrast with these germ-cells, which serve, not for the maintenance of the individual, but only for that of the species, all the other kinds of cells under the name of somatic or body-cells. The problem which we have to solve now lies before us in the question, How comes it that the germ-cell is able to bring forth from itself all the other cells in definite sequence and arrangement, and is thus able to build up the body of a new individual?

The similarity of this problem to that formulated in regard to unicellular organisms is at once obvious, but it becomes still more emphatic when we remember that the gulf between unicellular organisms and the higher animals and plants is bridged over by certain transition forms which are of the greatest interest, especially in relation to the problems of inheritance.