Chapter 2

In endeavoring to illustrate how this question actually affects research I will offer two illustrative cases, one of which may indicate the fruitfulness of the mechanistic conception in the analysis of complex and apparently mysterious phenomena, the other the nature of the difficulties that have in recent years led to attempts to re-establish the vitalistic view. The first example is given by the so-called law or principle of Mendel in heredity. The principle revealed by Mendel's wonderful discovery is not shown in all the phenomena of heredity and is probably of more or less limited application. It possesses however a profound significance because it gives almost a demonstration that a definite, and perhaps a relatively simple, mechanism must lie behind the phenomena of heredity in general. Hereditary characters that conform to this law undergo combinations, disassociations and recombinations which in certain way suggest those that take place in chemical reactions; and like the latter they conform to definite quantitative rules that are capable of arithmetical formulation. This analogy must not be pressed too far; for chemical reactions are individually definite and fixed, while those of the hereditary characters involve a fortuitous element of such a nature that the numerical result is not fixed or constant in the individual case but follows the law of probability in the aggregate of individuals. Nevertheless, it is possible, and has already become the custom, to designate the hereditary organization by symbols or formulas that resemble those of the chemist in that they imply the _quantitative_ results of heredity that follow the union of compounds of known composition. Quantitative prediction--not precisely accurate, but in accordance with the law of probability--has thus become possible to the biological experimenter on heredity. I will give one example of such a prediction made by Professor Cuénot in experimenting on the heredity of color in mice (see the following table). The experiment extended through three generations. Of the four grandparents three were pure white albinos, identical in outward appearance, but of different hereditary capacity, while the fourth was a pure black mouse. The first pair of grandparents consisted of an albino of gray ancestry, AG, and one of black ancestry, AB. The second pair consisted of an albino of yellow ancestry, AY, and a black mouse, CB. The result of the first union, AG x AB is to produce again pure white mice of the composition AGAB. The second union, AY x CB is to produce mice that appear pure _yellow_, and have the formula AYCB. What, now, will be the result of uniting the two forms thus produced--_i.e._ AGAB × AYCB? Cuénot's prediction was that they should yield eight different kinds of mice, of which four should be white, two yellow, one black and one gray. The actual aggregate result of such unions, repeatedly performed, compared with the theoretic expectation, is shown in the foregoing table. As will be seen, the correspondence, though close, is not absolutely exact, yet is near enough to prove the validity of the principle on which the prediction was based, and we may be certain that had a much larger number of these mice been reared the correspondence would have been still closer. I have purposely selected a somewhat complicated example, and time will not admit of a full explanation of the manner in which this particular result was reached. I will however attempt to give an indication of the general Mendelian principle by means of which predictions of this kind are made. This principle appears in its simplest form in the behavior of two contrasting characters of the same general type--for instance two colors, such as gray and white in mice. If two animals, which show respectively two such characters are bred together, only one of the characters (known as the "dominant") appears in the offspring, while the other (known as the "recessive") disappears from view. In the next generation, obtained by breeding these hybrids together, both characters appear separately and in a definite ratio, there being in the long run three individuals that show the dominant character to one that shows the recessive. Thus, in the case of gray and white mice, the first cross is always gray, while the next generation includes three grays to one white. This is the fundamental Mendelian ratio for a single pair of characters; and from it may readily be deduced the more complicated combinations that appear when two or more pairs of characters are considered together. Such combinations appear in definite series, the nature of which may be worked out by a simple method of binomial expansion. By the use of this principle astonishingly accurate numerical predictions may be made, even of rather complex combinations; and furthermore, new combinations may be, and have been, artificially produced, the number, character and hereditary capacity of which are known in advance. The fundamental ratio for a single pair of characters is explained by a very simple assumption. When a dominant and a recessive character are associated in a hybrid, the two must undergo in some sense a disjunction or separation in the formation of the germ-cells of the hybrid. This takes place in a quite definite way, exactly half the germ-cells in each sex receiving the potentiality of the dominant character, the other half the potentiality of the recessive. This is roughly expressed by saying that the germ-cells are no longer hybrid, like the body in which they arise, but bear one character or the other; and although in a technical sense this is probably not precisely accurate, it will sufficiently answer our purpose. If, now, it be assumed that fertilization takes place fortuitously--that is that union is equally probable between germ-cells bearing the same character and those bearing opposite characters,--the observed numerical ratio in the following generation follows according to the law of probability. Thus is explained both the fortuitous element that differentiates these cases from exact chemical combinations, and the definite numerical relations that appear in the aggregate of individuals.

Grandparents AG (white) AB (white) AY (white) CB (black) | | | | +---------+ +-----------+ | | Parents AGAB (white) AYCB (yellow) | | +----------------------+ | Observed Calculated {AGAY} {ABAY} (White) 81 76 {AGAB} Offspring ---------------{ABAB} { {AGCY} (Yellow) 34 38 {ABCY} { {ABCB (Black) 20 19 {AGCB (Gray) 16 19 ---- ---- 151 152

Now, the point that I desire to emphasize is that one or two very simple mechanistic assumptions give a luminously clear explanation of the behavior of the hereditary characters according to Mendel's law, and at one stroke bring order out of the chaos in which facts of this kind at first sight seem to be. Not less significant is the fact that direct microscopical investigation is actually revealing in the germ-cells a physical mechanism that seems adequate to explain the disjunction of characters on which Mendel's law depends; and this mechanism probably gives us also at least a key to the long standing riddle of the determination and heredity of sex. These phenomena are therefore becoming intelligible from the mechanistic point of view. From any other they appear as an insoluble enigma. When such progress as this is being made, have we not a right to believe that we are employing a useful working hypothesis?

But let us now turn to a second example that will illustrate a class of phenomena which have thus far almost wholly eluded all attempts to explain them. The one that I select is at present one of the most enigmatical cases known, namely, the regeneration of the lens of the eye in the tadpoles of salamanders. If the lens be removed from the eye of a young tadpole, the animal proceeds to manufacture a new one to take its place, and the eye becomes as perfect as before. That such a process should take place at all is remarkable enough; but from a technical point of view this is not the extraordinary feature of the case. What fills the embryologist with astonishment is the fact that the new lens is not formed in the same way or from the same material as the old one. In the normal development of the tadpole from the egg, as in all other vertebrate animals, the lens is formed from the outer skin or ectoderm of the head. In the replacement of the lens after removal it arises from the cells of the iris, which form the edge of the optic cup, and this originates in the embryo not from the outer skin but as an outgrowth from the brain. As far as we can see, neither the animal itself nor any of its ancestors can have had experience of such a process. How, then, can such a power have been acquired, and how does it inhere in the structure of the organism? If the process of repair be due to some kind of intelligent action, as some naturalists have supposed, why should not the higher animals and man possess a similar useful capacity? To these questions biology can at present give no reply. In the face of such a case the mechanist must simply confess himself for the time being brought to a standstill; and there are some able naturalists who have in recent years argued that by the very nature of the case such phenomena are incapable of a rational explanation along the lines of a physico-chemical or mechanistic analysis. These writers have urged, accordingly, that we must postulate in the living organism some form of controlling or regulating agency which does not lie in its physico-chemical configuration and is not a form of physical energy--something that may be akin to a form of intelligence (conscious or unconscious), and to which the physical energies are in some fashion subject. To this supposed factor in the vital processes have been applied such terms as the "entelechy" (from Aristotle), or the "psychoid"; and some writers have even employed the word "soul" in this sense--though this technical and limited use of the word should not be confounded with the more usual and general one with which we are familiar. Views of this kind represent a return, in some measure, to earlier vitalistic conceptions, but differ from the latter in that they are an outcome of definite and exact experimental work. They are therefore often spoken of collectively as "neo-vitalism."

It is not my purpose to enter upon a detailed critique of this doctrine. To me it seems not to be science, but either a kind of metaphysics or an act of faith. I must own to complete inability to see how our scientific understanding of the matter is in any way advanced by applying such names as "entelechy" or "psychoid" to the unknown factors of the vital activities. They are words that have been written into certain spaces that are otherwise blank in our record of knowledge, and as far as I can see no more than this. It is my impression that we shall do better as investigators of natural phenomena frankly to admit that they stand for matters that we do not yet understand, and continue our efforts to make them known. And have we any other way of doing this than by observation, experiment, comparison and the resolution of more complex phenomena into simpler components? I say again, with all possible emphasis, that the mechanistic hypothesis or machine-theory of living beings is not fully established, that it _may_ not be adequate or even true; yet I can only believe that until every other possibility has realty been exhausted scientific biologists should hold fast to the working program that has created the sciences of biology. The vitalistic hypothesis may be held, and is held, as a matter of faith; but we cannot call it science without misuse of the word.

When we turn, finally, to the genetic or historical part of our task, we find ourselves confronted with precisely the same general problem as in case of the existing organism. Biological investigators have long since ceased to regard the fact of organic evolution as open to serious discussion. The transmutation of species is not an hypothesis or assumption, it is a fact accurately observed in our laboratories; and the theory of evolution is only questioned in the same very general way in which all the great generalizations of science are held open to modification as knowledge advances. But it is a very large question what has caused and determined evolution. Here, too, the fundamental problem is, how far the process may be mechanically explicable or comprehensible, how far it is susceptible of formulation in physico-chemical or mechanistic terms. The most essential part of this problem relates to the origin of organic adaptations, the production of the fit. With Kant, Cuvier and Linnaeus believed this problem scientifically insoluble. Lamarck attempted to find a solution in his theory of the inheritance of the effects of use, disuse and other "acquired characters"; but his theory was insecurely based and also begged the question, since the power of adaptation through which use, disuse and the like produce their effects is precisely that which must be explained. Darwin believed he had found a partial solution in his theory of natural selection, and he was hailed by Haeckel as the biological Newton who had set at naught the _obiter dictum_ of Kant. But Darwin himself did not consider natural selection as an adequate explanation, since he called to its aid the subsidiary hypotheses of sexual selection and the inheritance of acquired characters. If I correctly judge, the first of these hypotheses must be considered as of limited application if it is not seriously discredited, while the second can at best receive the Scotch verdict, not proven. In any case, natural selection must fight its own battles.

Latter day biologists have come to see clearly that the inadequacy of natural selection lies in its failure to explain the origin of the fit; and Darwin himself recognized clearly enough that it is not an originative or creative principle. It is only a condition of survival, and hence a condition of progress. But whether we conceive with Darwin that selection has acted mainly upon slight individual variations, or with DeVries that it has operated with larger and more stable mutations, any adequate general theory of evolution must explain the origin of the fit. Now, under the theory of natural selection, pure and simple, adaptation or fitness has a merely casual or accidental character. In itself the fit has no more significance than the unfit. It is only one out of many possibilities of change, and evolution by natural selection resolves itself into a series of lucky accidents. For Agassiz or Cuvier the fit is that which was designed to fit. For natural selection, pure and simple, the fit is that which happens to fit. I, for one, am unable to find a logical flaw in this conception of the fit; and perhaps we may be forced to accept it as sufficient. But I believe that naturalists do not yet rest content with it. Darwin himself was repeatedly brought to a standstill, not merely by specific difficulties in the application of his theory, but also by a certain instinctive or temperamental dissatisfaction with such a general conclusion as the one I have indicated; and many able naturalists feel the same difficulty to-day. Whether this be justified or not, it is undoubtedly the fact that few working naturalists feel convinced that the problem of organic evolution has been fully solved. One of the questions with which research is seriously engaged is whether variations or mutations are indeterminate, as Darwin on the whole believed, or whether they may be in greater or less degree determinate, proceeding along definite lines as if impelled by a _vis a tergo_. The theory of "orthogenesis," proposed by Naegeli and Eimer, makes the latter assumption; and it has found a considerable number of adherents among recent biological investigators, including some of our own colleagues, who have made important contributions to the investigation of this fundamental question. It is too soon to venture a prediction as to the ultimate result. That evolution has been orthogenetic in the case of certain groups, seems to be well established, but many difficulties stand in the way of its acceptance as a general principle of explanation. The uncertainty that still hangs over this question and that of the heredity of acquired characters bears witness to the unsettled state of opinion regarding the whole problem, and to the inadequacy of the attempts thus far made to find its consistent and adequate solution.

Here, too, accordingly, we find ourselves confronted with wide gaps in our knowledge which open the way to vitalistic or transcendental theories of development. I think we should resist the temptation to seek such refuge. It is more than probable that there are factors of evolution still unknown. We can but seek for them. Nothing is more certain than that life and the evolution of life are natural phenomena. We must approach them, and as far as I can see must attempt to analyze them, by the same methods that are employed in the study of other natural phenomena. The student of nature can do no more than strive towards the truth. When he does not find the whole truth there is but one gospel for his salvation--still to strive towards the truth. He knows that each forward step on the highway of discovery will bring to view a new horizon of regions still unknown. It will be an ill day for science when it can find no more fields to conquer. And so, if you ask whether I look to a day when we shall know the whole truth in regard to organic mechanism and organic evolution, I answer: No! But let us go forward.

COLUMBIA UNIVERSITY PRESS

A Series of twenty-two lectures descriptive in untechnical language of the achievements in Science, Philosophy and Art, and indicating the present status of these subjects as concepts of human knowledge, are being delivered at Columbia University, during the academic year 1907-1908, by various professors chosen to represent the several departments of instruction.

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