The philosophy of biology

by James Johnstone

This transmission of a specific form and mode of behaviour from generation to generation is what a hypothesis of heredity attempts to explain—that is, to describe in the simplest possible terms, making use of the concepts of physical science. “Twelve years ago,” says Jacques Loeb, “the field of heredity was the stamping ground for the rhetorician and metaphysician; it is to-day perhaps the most exact and rationalistic part of biology, where facts cannot only be predicted qualitatively, but also quantitatively.” Let the reader examine for himself the meagre array of facts on which this apotheosis of mechanistic biology is based.

Two modern hypotheses of heredity demand attention—Weismann’s hypothesis of the continuity of the germ-plasm, and Semon’s “Mnemic” hypothesis. In the latter it is assumed that the basis of heredity is the unconscious memory of the organism: modes of functioning are “remembered” by the germ-plasm and are transmitted. This notion presents many points of similarity to that which we consider later on, so that it need only be mentioned here. Weismann’s hypothesis—like Darwin’s hypothesis of Pangenesis—is a corpuscular one, and has obviously been suggested by the modern development of the concepts of molecules and atoms in the physical sciences. It supposes that that which is handed down is a material substance of a definite chemical and physical structure. This is not the germ-cell, nor even the nucleus of the latter, but a certain material contained in the nucleus. The latter contains protein substances containing a greater proportion of phosphoric acid than does the cytoplasm of the cells in general; these proteins are known as nucleo-proteins, though our knowledge of their chemical structure is, so far, not very exact. It is not, however, these that are the germ-plasm, but a substance in the nucleus which becomes visible when the cell is killed in certain ways, and which becomes stained by certain basic dyes. It is distinguished by this character alone and on that account is loosely called “chromatin.” This substance Weismann identifies as “the material basis of inheritance.”

When a cell divides, a very complex train of events usually occurs. This process of “Mitosis” exhibits many variations of detail, and without actual demonstration it is rather difficult to explain clearly. But its essential feature is evidently the exact halving of all the structures in the cell which is about to divide. In the ordinary cell which is not going to divide immediately, the chromatin is diffused throughout the nucleus as very numerous fine granules, recognised only by their staining reactions. They may be concentrated at some part of the nucleus, so that a division through a plane of geometrical symmetry of the cell would not, in general, exactly halve the chromatin. Prior to division, therefore, this substance becomes aggregated as granules lying along a convoluted filament of a substance called “linin,” which is characterised principally by the fact that it does not stain with the dyes that stain the chromatin. The filament breaks up into short rods, called Chromosomes, and these rods become arranged in the equator of the nucleus. The rods then split longitudinally, and one-half of each moves towards one pole of the nucleus, the other half moving towards the other pole. Various other modifications of the cell and nucleus occur concomitantly with these changes, but the essential thing that happens seems to be the halving of all the structures of the cell, and this is the simplest explanation of the phenomena of mitotic cell division. Two daughter-cells are then formed by the division of the mother-cell, and each of these daughter-cells receives one-half of each of the chromatin granules that were contained in the mother-cell.

The chromosomes, or “Idants,” are seen to consist of discrete granules, and these are (generally) the bodies known as the “Ids.” The id cannot be resolved by the microscope into any smaller structures: it lies on the limits of aided vision; but the hypothesis assumes that it is composed of parts called “Determinants,” and the determinants are further supposed to consist of “Biophors.” The biophors are the ultimate organic units or elements, and they are of the same order of magnitude as chemical molecules. We must suppose them to be more complex than a protein molecule, and the latter contains many hundreds (at least) of chemical atoms. Now it is possible to calculate the number of atoms contained in a particle of the same size as the id: such a calculation may be made by different methods, all of them yielding concordant results. This calculated number of atoms may be less than that which we must suppose to be present in the biophors, of which the hypothetical id is composed![28]

The id is supposed to contain all the potentialities of the completely developed organism. It is composed of a definite number of determinants, each of the latter being a “factor” for some definite, material constituent of the adult body. There would be a determinant for each kind of cell in the retina of the eye, one for the lens, one for the cornea (or rather for each kind of tissue in the latter), one for each kind of pigment in the choroid and iris, and so on; every particular kind of tissue in the body would be represented by a determinant. Thus packed away in a particle which lies just on the limits of microscopic vision are representatives of all those parts of the body which are chemically and physically individualised, each of these hypothetical “factors” being a very complex assemblage of chemical atoms. In development the determinants become separated from each other, so that whatever parts of the body are formed by the first two blastomeres are represented by determinants which are contained in those cells, and which are sifted out from each other and segregated. As development proceeds this process of sifting becomes finer and finer, until when the rudiments of each kind of tissue have been laid down a cell contains only one kind of determinant. This consists of biophors of a special kind, and the latter then migrate out from the chromatin into the cytoplasm of the cells in which they are contained, and proceed to build up the particular kind of tissue required.

The nucleus of the germ-cell is thus a mixture of incredible complexity, but in addition to this material mixture there must exist in it the means for the arrangement of the determinants in the positions relative to each other occupied by the adult organs and tissues. A mechanism of unimaginable complexity would be required for this purpose, and it must be a mechanism involving only known chemical and physical factors. It is safe to say that absolutely no hint as to the nature of this mechanism is contained in the hypothesis.

The determinants must be able to grow by reproduction, or by the accretion of new biophors, since in each generation new germ-cells are formed. If we say that they grow by reproduction in the sense that an organism grows by reproduction, we beg the question of their means of formation. Do they grow by the addition of similar substances in the way that a crystal grows? If so, the molecules of which they are composed must exist in the lymph stream bathing the germ-cells—that is, the biophors themselves must already exist in this liquid, for if we suppose that the biophors are able to divide and grow by making use of the protein substances which we know are present in the lymph stream, then we confer upon these bodies all the properties of the fully developed organism. If they are present in the blood, then the composition of the latter must be one of inconceivable complexity, since it must contain as many substances as there are distinct tissues in the animal body. We know, of course, that this is not the case. How, then, are the biophors reproduced?

We must leave this field of unbridled speculation (which cannot surely be “the most exact and rationalistic part of biology.”) What the study of the reproduction of the organism does show is that something—which we call the specific organisation—is handed down from parent to offspring, and that this something may possess a high degree of stability. No apparent change of significance can be observed in the very numerous generation of organisms (the 2000 generations of Paramœcium, for instance, which were bred by Woodruff) which can be produced by experimental breeding. Some species of animals—the Brachiopod Lingula, for instance—have persisted unchanged since Palæozoic times. Throughout the incredibly numerous generations represented by this animal series, the specific organisation must have been transmitted in an almost absolutely unchanged condition. The germ-plasm is therefore continuous from generation to generation, and it possesses an exceedingly great degree of constancy of character. This conception of the continuity and stability of the specific organisation is the feature of value in Weismannism, and all that we know of the phenomena of heredity confirms it. But it is pure speculation to regard the organisation as an aggregate of chemically distinct substances, or if we say that this speculation is rather a working hypothesis, then it must justify itself by leading us back again to the results of experience.

It is, however, not quite accurate to say that the organisation persists unchanged from generation to generation. The offspring is similar to the parent—that is, the organisation has been transmitted unchanged. But the offspring also differs just a little from the parent—that is to say, the organisation is modified by each transmission. In these two statements we formulate in the simplest manner the law of organic variability. Organisms may obviously be arranged in categories in such a way that the individuals in any one category resemble each other more closely than they resemble the individuals belonging to another category. We may, by experimental breeding, produce an assemblage of organisms all of which have had a common ancestor, or a pair of ancestors. Now the individuals composing such an assemblage would exhibit a close resemblance to each other, such a resemblance as our categories of naturally occurring organisms are seen to exhibit. We should also find that the individuals of our naturally occurring assemblage would be able to interbreed among themselves, just as in the case of the experimentally produced population. It may be concluded, then, that the naturally occurring population is also the product of a pair of ancestors. This inter-fertility, as well as the close morphological resemblance of the individuals, are the facts on which the hypothesis of the common origin and unity of the assemblage, or species, is formed.