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Twenty years ago in describing the facts of Variation, argument was necessary to show that these phenomena had a special value in the sciences of Zoology and Botany. This value is now universally understood and appreciated. In spite however of the general attention devoted to the study of Variation, and the accumulation of material bearing on the problem, no satisfactory or searching classification of the phenomena is possible. The reason for this failure is that a real classification must presuppose knowledge of the chemistry and physics of living things which at present is quite beyond our reach.

It is however becoming probable that if more knowledge of the chemical and physical structure of organisms is to be attained, the clue will be found through Genetics, and thus that even in the uncoordinated accumulation of facts of Variation we are providing the means of analysis applicable not only to them, but to the problems of normality also.

The only classification that we can yet institute with any confidence among the phenomena of Variation is that which distinguishes on the one hand variations in the processes of division from variations in the nature of the substances divided.

Variations in the processes of division are most often made apparent by a change in the number of the parts, and are therefore called Meristic Variations, while the changes in actual composition of material are spoken of as Substantive Variations. The Meristic Variations form on the whole a natural and fairly well defined group, but the Substantive Variations are obviously a heterogeneous assemblage.

Though this distinction does not go very far, it is useful, and in all probability fundamental. It is of value inasmuch as it brings into prominence the distinct and peculiar part which the process of division, or, more generally, repetition of parts, plays in the constitution of the forms of living things.

That there may be a real independence between the Meristic and the Substantive phenomena is evident from the fact both that Meristic changes may occur without Substantive Variation, and that the substances composing an organism may change without any perceptible alteration in its meristic structure. When the distinction between these two classes of phenomena is perceived it will be realised that the study of genetics has on the one hand a physical, or perhaps more strictly a mechanical aspect, which relates to the manner in which material is divided and distributed; and also a chemical aspect, which relates to the constitution of the materials themselves. Somewhat as the philosophers of the seventeenth and eighteenth centuries were awaiting both a chemical and a mechanical discovery which should serve as a key to the problems of unorganised matter, so have biologists been awaiting two several clues. In Mendelian analysis we have now, it is true, something comparable with the clue of chemistry, but there is still little prospect of penetrating the obscurity which envelops the mechanical aspect of our phenomena. To make clear the application of the terms chemical and mechanical to the problem of Genetics the nature of that problem must be more fully described. In its most concrete form this problem is expressed in the question, how does a cell divide? If the organism is unicellular, and the single cell is the whole body, then the process of heredity is accomplished in the single operation of cell-division. Similarly in animals and plants whose bodies are made up of many cells, the whole process of heredity is accomplished in the cell-divisions by which the germ-cells are formed. When therefore we see a cell dividing, we are witnessing the process by which the form and the properties of the daughter-cells are determined.

Now this process has the two aspects which I have called mechanical and chemical. The term "Entwicklungsmechanik" has familiarised us with the application of the word mechanics to these processes, but on reflexion it will be seen that this comprehensive term includes two sorts of events which are sometimes readily distinguishable. There is the event by which the cell divides, and the event by which the two halves or their descendants are or may be differentiated. It is common knowledge that in some cell-divisions two similar halves, indistinguishable in appearance, properties, and subsequent fate, may be produced, while in other divisions daughter-cells with distinct properties and powers are formed. We cannot imagine but that in the first case, when the resulting cells are identical, the division is a mechanical process by which the mother-cell is simply cut in two; while in order that two differentiated halves may be produced, some event must have taken place by which a chemical distinction between the two halves is effected.[1] In any ordinary Mendelian case we have a clear proof that such a chemical difference may be established between germ-cells. The facts of colour-inheritance for instance prove that germ-cells, otherwise identical, may be formed possessing the chromogen-factor which is necessary to the formation of colour in the flowers, or destitute of that factor. Similarly the germ-cells may possess the ferment which, by its action on the chromogenic substance, produces the colour, or they may be without that ferment. The same line of argument applied to a great range of cases. Nevertheless, though differences in chemical properties are often thus constituted by cell-divisions, and though we are thus able to make a quasi-chemical analysis of the individual by determining and enumerating these properties, yet it is evident that the distribution of these factors is not itself a chemical process. This is proved by the fact that similar divisions may be effected between halves which are exactly alike, and also by the fact that the numbers in which the various types of germ-cells are formed negative any suggestion of valency between them. The recognition of the unit-factors may lead—indeed must lead—to great advances in chemical physiology which without that clue would have been impossible, but in causation the chemical phenomena of heredity must be regarded as secondary to the physical or mechanical phenomena by which the cells and their constituents are divided and separated. When therefore we speak of the essential phenomena of heredity we mean the mechanics of division, especially, though not, as we shall see, exclusively, of cell-division; and in the relation between the two halves of the dividing cell we have the problem presented in what seems to be its simplest form.

In attempting to form some conception of the processes by which bodily characteristics are transmitted, or—to avoid that confusing metaphor of "transmission"—how it comes about that the offspring can grow to resemble its parent, continuity of the germ-substance which in some animals is a visible phenomenon,[2] gives at least apparent help. An egg for example on becoming adult develops in certain parts a particular pigment. The eggs of that adult when they reach the appropriate age develop the same pigment. We have no clear picture of the mechanism by which this process is effected, but when we realise that the pigment results from the interaction of certain substances, and that since all the eggs are in reality pieces of the same material, it seems, unless we inquire closely, not unnatural that the several pieces of the material should exhibit the same colours at the same periods of their development. The continuity of the material of the germs suggests that there is a continuity of the materials from which the pigment is formed, and that thus an actual bit of those substances passes into each egg ready at the appropriate moment to generate the pigment. The argument thus outlined applies to all substantive characteristics. In each case we can imagine, if we will, the appearance of that characteristic as due to the contribution of its rudiment from the germ tissues.

When we consider more critically it becomes evident that the aid given by this mental picture is of very doubtful reality, for even if it were true that any predestined particle actually corresponding with the pigment-forming materials is definitely passed on from germ to germ, yet the power of increase which must be attributed to it remains so incomprehensible that the mystery is hardly at all illuminated.

When however we pass from the substantive to the meristic characters, the conception that the character depends on the possession by the germ of a particle of a specific material becomes even less plausible. Hardly by any effort of imagination can we see any way by which the division of the vertebral column into x segments or into y segments, or of a Medusa into 4 segments or into 6, can be determined by the possession or by the want of a material particle. The distinction must surely be of a different order. If we are to look for a physical analogy at all we should rather be led to suppose that these differences in segmental numbers corresponded with changes in the amplitude or number of dividing waves than with any change in the substance or material divided.