The phenomena of alternation of generations require the supposition that in those animals and plants in which it occurs 'two kinds of germplasm exist, both of which always are present in the egg or in the bud, but of which one only is active at any time and rules the ontogeny, while the other remains inactive.' The alternating activity of these two produces the alternation of generations. So also dimorphism, which is exhibited most frequently as differences between the sexes, is explained by the assumption that 'double determinants' are present in the germplasm for all the cells, cell-groups, or entire organisms which have different characters in the male and female.
One set of these double determinants remains latent, the other becomes active.
Finally, to explain the phenomena of regeneration, it is assumed that in the complicated cases where large parts of the body, like the head, the tail, or a bone, can be replaced after accidental loss, the cells with this power of regeneration contain, in addition to the determinants proper to them, _supplementary determinants_, which contain the germs needed for regeneration of the lost parts. These were handed on, during the ontogeny, through definite series of cells, in a passive condition, to become active when the conditions for their growth are supplied by the loss of the parts they can replace.
CRITICISM OF THE GERMPLASM THEORY.[8]
At first sight, much of Weismann's fabric of hypotheses gives the impression of being a closed system, thought out as a whole, and it has been treated as such in most of the notices and criticisms which I have seen. As a matter of fact, Weismann has spared no pains in the elaboration of his system, and has attempted to bring under his theory the many different phenomena of heredity and development, as well as alternation of generations, regeneration, atavism, and so forth. But, on the other hand, he has been careless in testing the stability and security of the foundations upon which he has built. It is on solid foundations that lie deep in the earth, and that avoid all reproach of being scamped or superficial work, that the durability of a structure depends. In this criticism the details of the superstructure will be disregarded, but the foundation will be tested thoroughly.
Cells and cell-properties are essential parts of Weismann's theory; while Naegeli has attempted to make his theory of the idioplasm independent of the whole conception of cells. In this matter I agree with Weismann, as, indeed, with De Vries and others, and I consider that the course taken by Naegeli has made his position untenable.
Naegeli would make his theory of the idioplasm quite independent of the theory of cells, because, while cells are important units in morphological structure, independently of this they cannot be regarded as important units. 'By a unit,' he insists, 'we must understand, in a physical sense, a system of material particles. In the organic world there are very many kinds of higher and lower units; vegetable and animal individuals, organs, tissues, groups of cells (in the vegetable kingdom, for instance, vessels and sieve-tubes), cells, parts of cells (plant cell-membranes, plasma, granules, and crystalloids, starch--grains, fat-globules, and so forth), micellae, molecules, atoms. In morphology and physiology, sometimes one kind of unit, sometimes another, comes characteristically and notably into evidence. That being so, there is no reason why a special kind of unit should be exalted in a general theory.'
Although, with Naegeli, we must recognise and keep in view the presence of a large number of higher and lower units in the organic world, a fact upon which I shall lay considerable emphasis later, we must none the less recognise that, among all elementary units, cells are most the conspicuous, morphologically and physiologically, in the whole organic realm. In actual research this is avowed very practically, as a glance at the biological literature of the last thirty years will show. Especially in the study of heredity, the cell is a unit that cannot be neglected, for it has been established that spores, ova, and spermatozoa, the units by which species are preserved in reproduction, both in the animal and in the vegetable kingdom, have the morphological value of cells.
In this point I am in opposition to Naegeli, although otherwise I agree with much in his conceptions.
A theory of heredity must be reconciled with the cell theory. In investigating Darwin's pangenesis, Galton's doctrine of the stirp, Naegeli's idioplasm, Weismann's germplasm, the intracellular pangenesis of De Vries, His' doctrinal of germinal foci for the formation of organs, or Roux's mosaic theory, I believe that one must face the question: How far do these doctrines agree with what we know about the structure and function of the cell? Moreover, in deciding between the alternatives--preformation and epigenesis--I believe that it will profit us to start our critical investigation with the cell itself. With this object, I shall now sum up in a few sentences as much of our present knowledge of the life of cells as, I believe, must be reckoned with in any theory of propagation.
The cell, which consists of protoplasm and a nucleus, is an elementary organism, that, by itself, or in combination with other cells, forms the basis of all animal and vegetable organisation. In minute structure it is so extraordinarily complicated that its essential constitution (its micellar or molecular structure) eludes our observation. It is a medley, composed of numerous, chemically distinct particles that may be divided into two groups, organised and unorganised. The latter are free, or in solution; they are such as albuminates, fats, carbo-hydrates, water, salts, and they serve as material for the nutrition and growth of the cell.
The former make up the living cell body (in the narrow sense). They are able to multiply by growth and division, and they are therefore the elementary parts, units of life of lower rank, of which the cell, a unit of higher rank, is composed. They are the gemmules of Darwin, the physiological units of Spencer, the bioblasts of Altmann, the pangenes of De Vries, the plasomes of Wiesner, the idioblasts of Hertwig, and the biophores of Weismann.
The cells of every organic species possess a proper, specific organisation, more or less complicated, and, in correspondence with this, they are composed of more or less numerous and varied organised particles.
The nucleus is a special organ of cells, which is always present. It displays a collection of numerous, peculiar, elementary living units, the idioblasts. These show chemical, morphological, and functional differences from the plasomes, the living units of the protoplasm; but perhaps the idioblasts, by absorption of different material, may transform themselves into the plasomes, just as these last, by a similar process, may produce the plasma-products. In my view, the nucleus is the bearer of the idioplasm or hereditary material, that is to say, of a substance that is more stable than protoplasm, and, because it is less subject to influences of the outer world, it stamps its specific character upon the organism.[9]
A mass of protoplasm with several nuclei (like the myxomycetes, coeloblasts, etc.) has the morphological value of a number of cells (synergides), corresponding to the number of the nuclei.
The means by which the continuity of life is maintained is the capacity of the cell to manifold itself by division, so forming two or more separate pieces. The process, which in most cases is associated with complicated changes of the nuclear contents, appears essentially to consist of the following: The elementary units of the cell (centrosomes, chromatin bodies in the case of nuclear division), being endowed with special energy resulting from the processes of growth, divide, and the elementary products of division separate into two groups, which move from the middle line; upon this there follows a division of the general body of the cell, _i.e._, of the protoplasm and its contents.
From the point of view of cells, I believe myself compelled to raise several objections to most important bases of Weismann's germplasm theory.
For convenience of exposition these may be divided into two groups: Objections to the hypothesis of differentiating division; objections to Weismann's doctrine of determinants.
I. OBJECTIONS TO THE HYPOTHESIS OF DIFFERENTIATING DIVISION.
A corner-stone of Weismann's theory is his assumption of nuclear divisions which are differentiating. Proof of this fundamental assumption may be sought in vain in Weismann's writings. Instead of that, a series of abstract arguments are brought forward in favour of it. Thus on p. 31 (of the English translation) Weismann treats the chromatin in the nucleus of the fertilised egg as the substance which accomplishes inheritance, and he denotes all the nuclei of the organism arising from the nucleus of the egg by divisions as the chromatin-tree, and then goes on to ask whether or no the pieces of hereditary material that make up the chromatin-tree of an organism are like each other or different. 'It can easily be shown,' the answer runs, 'that the latter must be the case.' For 'the chromatin is in a condition to impress the specific character on the cell in the nucleus of which it is contained. As the thousands of cells which constitute an organism possess very different properties, the chromatin which controls them cannot be uniform; it must be different in each kind of cell.'
Moreover, on p. 45 (of the English edition), 'The fact itself' (the capacity on the part of the idioplasm for regular and spontaneous change) 'is beyond doubt. When once it is established that the morphoplasm of each cell is controlled, and its character decided, by the idioplasm of the nucleus, the regular changes occurring in the egg-cell, and the products of its division in each embryogeny, must then be referred to the corresponding changes of the idioplasm.'
Finally, on p. 205 (of the English edition), 'The cells of the segmenting ovum are completely dissimilar as regards their hereditary value, although they are all young and embryonic, and are not infrequently quite similar in appearance. It therefore seems to me to follow from this, as a logical necessity, that the hereditary substance of the egg-cell, which contains all the hereditary tendencies of the species, does not transmit them _in toto_ to the segmentation cells, but separates them into various combinations, and transmits them in groups to the cells. I have taken account of these facts in considering the regular distribution of the determinants of the germplasm, and the conversion of the latter into the idioplasm of the cells in the different stages of ontogeny.'
In the different propositions I have quoted, we have to deal with what is merely a fallacy in rhetorical disguise. For, from the premiss that the chromatin has the power of impressing specific character upon the protoplasm of the cell, it by no means follows that two cells, distinguishable by the nature of their plasma-products, must therefore contain different kinds of protoplasm. There are other possibilities to be reckoned with. Weismann himself knows that there is no logical necessity for the conclusion, for he himself suggests another possibility in the following: 'If we wished to assume that the whole of the determinants of the germplasm are supplied to all the cells of the entogeny, we should have to suppose that differentiation of the body is due to all the determinants except _one_ particular one remaining dormant in a regular order, and that, apart from special adaptations, only one determinant reaches the cell, viz., that which has to control it. If, however, we do make the assumption,' etc. (p. 63, English edition).
Here, then, Weismann himself points out that what in other places he has attempted to represent as a necessary conclusion is but one of two alternatives.
Not only does he grant the possibility of the alternative, but uses it himself in explanation of the phenomena of reproduction and development. He attributes to certain series of cells, in addition to the active rudiments controlling the normal characters of their protoplasm, the possession of numerous latent rudiments which become active when opportunity presents itself.
This _non sequitur_ in his argument Weismann excuses with the remark that the presence of latent rudiments in special cases 'depends, as I believe, upon special adaptations, and is not primitive, at any rate not in higher animals and plants. Why should Nature, who always manages with economy, indulge in the luxury of always providing all the cells of the body with the whole of the determinants of the germplasm, if a single kind of them is sufficient? Such an arrangement will presumably have occurred only in cases where it serves definite purposes' (p. 63, English edition). Here, again, is a rhetorical flourish instead of a proof.
But the dilemma which we are examining is not yet at an end. Supposing for the moment that we accept the assumption that different character in cells implies different character in their nuclear matter, we have at once a new and important decision to make. Does the nuclear matter in the different cells, that has arisen by division from the nuclear matter of the egg-cell, become unlike by the process of division itself? or is it only after the division that it becomes different, and in consequence of the action of outer forces upon the nuclei?
Weismann decides boldly--but again without bringing forward proof--in favour of the former interpretation. 'For the chromatin,' he remarks,[10]
'cannot _become_ different in the cells of the fully formed organism; the differences in the chromatin controlling the cells must begin with the development of the egg-cell and must increase as development proceeds; for otherwise the different products of the division of the egg-cell could not give rise to entirely different hereditary tendencies. This is, however, the case.' Weismann represents to himself that[11] 'the changes of the idioplasm depend on purely internal causes, which lie in the physical nature of the idioplasm. In obedience to these, a division of the nucleus accompanies each qualitative change in the idioplasm, in which process the different qualities are distributed between the two resulting halves of the chromatin rods.'
I shall proceed to show that this conception involves material difficulties and contradictions. It will be found that characters totally contradictory are ascribed to Weismann's idioplasm. On the one hand, it is credited with being a stable substance, possessing a coherent, complicated architecture; in the form of ancestral plasms it is supposed to be handed on, from one individual to another, unchanged through many generations; on the other hand there is ascribed to it a labile architecture, that allows a free and perpetual casting loose of rudiments, of such a kind that at each division there is caused a complete rearrangement and unequal division of these rudiments. In the one case, the inner forces produce a reciprocal, coherent bond between the numerous rudiments; in the other case, permit change of their position and relations to one another, and this not only once but in orderly, definite fashion, different in each of many successive divisions, so that the _id_ comes to possess a completely altered architecture. 'Each _id_ in every stage' (p. 77 of the English edition), has its definitely inherited architecture; its structure is a complex, but a perfectly definite one, which, originating in the _id_ of germplasm, is transferred by regular changes to the subsequent idic stages. The structure exhibited in all these stages exists potentially in the architecture of the _id_ of germplasm: to this architecture is due, not only the regular distribution of the determinants--that is to say the entire construction of the body from its primary form.'
Unfortunately, Weismann's hypothesis tells us nothing at all about these internal causes, that depend upon the physical nature of the idioplasm; that is to say, nothing at all about the causes which, working in a fashion so contradictory and astonishing, really produce the whole development.
In such a state of affairs it is better to turn to Nature herself; and to see whether or no the occurrence of differentiating division of the nucleus in the organic world is at all supported by the actual observations and investigations of those who study cells.
We shall examine (1) Unicellular organisms; (2) Lower multicellular organisms; (3) The phenomena of generation and regeneration; (4) alteration of structural growth due to external interferences (heteromorphosis); (5) A number of physiological indications that cells and tissues, in addition to their patent characters, contain latent characters which have reached them by doubling division, and which are representative of the species.
FIRST GROUP OF FACTS.--UNICELLULAR ORGANISMS.
Doubling division alone exists, or could exist, among unicellular organisms. The maintenance of the species depends upon this. Our belief that a species produces only its own species, that like begets only like, a belief that finds continual confirmation all through the study of systematic and embryological natural history, would disappear, were it possible that in the division of unicellular organisms the hereditary mass should be split into two unequal components and be bestowed unequally upon the daughter-cells. All research shows that unicellular fungi, algae, infusoria, and so forth, in dividing, transmit specific characters so strongly and in detail so minute that their descendants, a million generations off, resemble them in every respect. No one has doubted the fact, and Weismann himself recognises that division, among unicellular organisms, is always doubling. The process of division, as such, appears never to be the means by which new species are called into existence among unicellular organisms. This is a fundamental proposition of cell-life, not to be doubted, and to be taken into account in the presentation of theories of heredity.
From the proposition that like begets only like the corollary by no means follows that mother- and daughter-cells must appear identical from the beginning. For the identity under consideration belongs only to the substance that is the bearer of specific characters, to the hereditary mass; besides that, a unicellular organism contains other substances, substances that change from time to time during its life. Many unicellular organisms pass through a regular series of developmental stages; the stages themselves being inherited, and following each other as infallibly as in the case of embryonic stages of higher animals.
The following will serve as examples of this. _Podophrya gemmipara_, an Acinetan, in the adult condition is attached by a long stalk, while the free end, at which is the mouth, is provided with suctorial tentacles. It reproduces by giving rise to many little buds, ciliated on the upper surface like free-swimming, hypotrichous infusoria. These, in appearance, are quite unlike the parent organism, and, after a vagrant existence in the water for some time, they attach themselves to a surface and produce a stalk, tentacles with suctorial pseudopodia, and so for the first time attain the maternal form.
Some Gregarines are large, jointed cells, divided into two pieces, a protomerite and a deutomerite; they are clad with a cuticle, under which lies a layer of muscular fibrils. After conjugation they encyst, the nucleus divides, and they break up into numerous peculiarly-shaped boat-like structures, (pseudonavicellae), which afterwards are set free as small, sickle-shaped embryos. These exceedingly small germ-cells afterwards develop into the very different, adult gregarine-cells.
If the characters of a species be associated with a hereditary mass, an actual substance that is handed on from the parent-cell to the offspring, it is clear that the infusoria-like vagrant young of the Acinetan, and the sickle-shaped embryos of the Gregarine possess it, although for some time they are quite unlike the parent organism. For at last they become an Acinetan or a Gregarine, exactly like the parent-cell from which they arose as embryos.
These circumstances, among unicellular organisms, are a weighty indication of the error of concluding, with Weismann, in the case of multicellular forms, that because cells are unlike in outward appearance, the hereditary mass, or, as I call it, the nuclear matter, within them is also unlike.
Such an assumption would involve us in the greatest contradictions. For the supposition that the nucleus is the hereditary mass transmitting the characters of the species necessitates the conclusion, in the case of unicellular forms, that the hereditary mass remains in possession of all the rudiments of the cell while it passes through the various phases of its cycle of development. Otherwise, these phases would have to be acquired anew in each case. We must, therefore, represent the possibilities of exchange between the nucleus, in its capacity of bearer of the hereditary mass, and the protoplasm as being such that all the rudiments are not simultaneously in activity, but that some of them can remain latent for a time.
SECOND GROUP OF FACTS.--THE LOWER MULTI-CELLULAR ORGANISMS.
Although in the development of unicellular organisms the way by which like begets like is plain and intelligible enough, at least in the cases dealt with, it is different with multicellular organisms, which have reached a higher grade of development. Among them we have to do with a continuous process of development, in which the highly-differentiated, multicellular organism arises from an egg, and in turn gives rise to an egg, and so on in unending sequence. But the succeeding stages of the sequence are so exceedingly dissimilar in appearance that the question how one step of the series turns into the next, and, above all, the question how the similarity of organisms, separated by the egg-stage, can be transmitted through the egg-stage, form the deepest riddle offered to biological investigation.
Here, in a completeness so wonderful that our intelligence can hardly apprehend it, are presented to us the qualities of the organic material of which cells are made. Here lies that dark secret into which the various theories of generation try to direct a beam of light, and seek to find out the direction in which explanation may be found.
An intermediate stage which may serve towards the explanation of these circumstances is presented by the lower multicellular organisms, such as threadlike algae, fungi, and other simple creatures. In them cells arise by division from the egg or from the spore, and become united into an individual of a higher rank; these cells resemble one another so completely in appearance and in qualities that there can be as little doubt as in the case of unicellular organisms that they arose by doubling division.
It is certain, then, that there exist multicellular bodies, often consisting of many thousand cells, in which each part retains the qualities of the egg from which it arose by doubling division, and which, as that method implies, possess the rudiments of the whole of which each is a part.
In this category there naturally fall the multinucleated masses of protoplasm, sometimes highly organised, in which every nucleus, surrounded by a shell of protoplasm, is capable of reproducing the whole. I am thinking of the slime-fungi (_Myxomycetes_), with their peculiar formation of reproductive bodies; of the 'acellular plants,' which in some cases closely resemble multicellular species in their formation of leaf and root, and in their mode of growth, as, for instance, _Caulerpa_, the multinucleated _Foraminifera_ and _Radiolarians_. For, according to our definition of the cell, a multinucleated organism potentially is a multicellular organism.
In this matter Weismann has assumed a position which leads to peculiar consequences. In his opinion, somatic cells and germ-cells were sharply distinct at their first appearance in evolution, and have remained so ever since. Transitional forms between them are nowhere to be found. It would be inconsistent with his theory of the germplasm had somatic cells contained germplasm as their idioplasm, even when the soma first came into existence. The phyletic origin of the somatic cells depended directly upon an unequal separation of the determinants contained in the germplasm. It would totally contradict his presentation if the somatic cells, even at their first origin in phylogeny, contained, in addition to their patent special qualities, the qualities common to the whole species in a latent condition.
Weismann's conception, therefore, implies that many of the lower multicellular organisms, having no somatic-cells, have no body. Take the closely-allied creatures _Pandorina morum_ and _Volvox globator_, which Weismann himself brings forward as instances for his view; the latter has a body, the former has no body, as all its cells are able to serve for reproduction!
It is enough to have pointed out how contradictory are the interpretations in this matter. Enlarging upon them may be postponed at present, for we are concerned here not with the interpretation of individual cases, but with the principles involved in the question, and, therefore, we must pass on to show further reason for considering the existence of differentiating division highly improbable in the whole organic world.