Science : Yesterday and To-day
[The following article is the fourth of a series, not mainly intended to convey lmowledge of particular conclusions that are being reached in various sciences—this will only be incidental—but rather to give some conception of the new modes of thought and changes of method that are being developed with the extension of scientific knowledge, in a manner which is comprehensible and interesting to the lay reader. This week Professor Julian Huxley, of King's College, writes on "Recent Progress in Biology." Next week Professor A. M. Carr Saunders will write on " Sociology."]
Recent Progress in Biology
BY PROFESSOR JULIAN HUXLEY.
THEgreatest change which has come over biology in the present century is its unification. Thirty years ago evolution had become more or less of an armchair subject, against whose academic sterilities many of the most original minds in biology were in rebellion. There was as yet no plausible theory of heredity. What is now the science of Animal Behaviour had just emerged from the anecdotal stage. Comparative Anatomy had become a formidable but isolated discipline, which not infrequently (I am quoting from Radl's History of Bio- logical Theory) was so busy comparing one thing with another that it sometimes forgot, to ask what either of them really was. Sex-determination was still a mystery. The idea of hormones had not been properly formulated by the physiologists, and had not penetrated at all to general biology. The physiology of development was in its infancy ; so was the science of comparative physiology. The study of function, when it was not anthropocentric and medical, was (if I may coin a word) batrachocentric, for it dealt almost wholly with frogs ; and there was little interpenetration between Departments of Physiology and Zoology. . Cytology, the study of cells and their miniature organs such as chromosomes, was well developed, but had made few contacts with other branches of Biology. And Systematics was quite content to go on piling up new species (or at least new specific names) and to fill in minor details as to geographical distribution.
The advertisements of Mazawattee Tea bid us recall the Delicious Blends of Thirty Years Ago. In Biology thirty years ago the matter was that there had been so little blending, delicious or otherwise ; a number of separate sub-sciences lay about, so to speak, some hoping for a happier fate in reunion, others self-sufficiently con- tented with their isolation.
To-day, how different is the picture ! Biology is a unified science. It is still, of course, full of gaps, ob- viously incomplete on, every hand ; but its different branches have sprouted and become organically united. It is now possible to teach biology as a connected whole. Some universities have even begun to do so.
The transformation has not been due to any one factor. The rediscovery of Mendel's work in 1900 was the first vital stimulus, for through it not only was a new sub- science of heredity created, but at a bound the micro- scopical study of cells came. o be linked with the study of experimental matings in the breeding-pen ; the chromo- somes turned out to be the microscopic agents of Mendel's laws. But expansion in many directions was necessary
before inter-connexion could be effected ; and the best I can do with my remaining space is not to attempt a his- torical survey, but to try to explain something of the present structure of this new unified biology.
In first place comes the marriage of genetics with evolution, a union destined to be fertile indeed. The happy ending is interesting, for at first the discoveries of Mendelism only seemed to accentuate existing cleavages. Good Darwinians denied that the characters with which Mendelians dealt had anything to do with evolution ; good Mendelians scoffed at the efficacy of Natural Selection. Since those early days, however, much water has flowed under our bridges. In the first place, Mendelism has been generalised. All its laws spring from two facts : that the physical basis of heredity consists of self-repro- ducing material units (the so-called genes), and that these units are strung together in groups within the visible bodies known as chromosomes. Secondly, the behaviour of genes appears to account for the hereditary transmis- sion of all save a few minor characters. Thirdly, the genes, though normally stable, are found occasionally to alter (mutate) into a new and equally stable phase ; and mutation has how been artificially produced on a large scale by means of X-rays. Fourthly, whereas at first only " large " mutations, producing striking effects, were known, intensive study has revealed that " small " muta- tions are more numerous, and also more important as raw material for evolution, since they may be beneficial to their possessors, while large ones are usually upsetting in their effects. And fifthly, mathematical methods have been applied to the problem.
The net result is that Selection and Mutation are now reconciled, and we can all be Darwinians again, though we must wear our Darwinism with a difference. Small muta- tions turn out to be the raw material without which evolutionary change would not be possible ; but selection is the main agent which directs and guides that change. The extent of our progress may be gauged by Dr. R. A. Fisher's new and very important book, The Genetical Basis of Natural Selection. In his first chapter he points out that the older Darwinians wanted to reject the ideas of Mendelism as inconsistent with their scheme ; but he proceeds to demonstrate the startling fact that without the Mendelian postulates of independent and relatively stable hereditary units, natural selection could never get a grip on the constitution of the species, and, to be brief, would not work !
The corollary of the idea that small mutations are the chief raw material for evolution is that evolution must usually be a gradual process. This has been put beyond reasonable doubt, as a fact in its own right, by the recent work both of palaeontologists and of systematists. The describers of fossils have now given us series of extinct forms which would have brought tears of joy to Darwin's eyes, series in which gradual change in definite directions is always to be seen. Such series have been unearthed (in the literal sense of the word) among horses, sea-urchins, camels, snails, titanotheres, starfish, sabre-tooths, ammon- ites, elephants and other creatures.
The systematists meanwhile have devoted a great deal of attention to studying minor variation within the species, and other aspects of the species problem. As a result, we now can point to local Varieties which differ from the neighbouring type in all degrees, from scarcely perceptible nuances up to distinctions obviously of specific rank. Isolation, we can say with assurance, un- doubtedly tends to lead to divergence, and the divergence is generally a gradual cne. So that the observational testimony of these sub-sciences is at one with the experi- mental and analytic studies of genetics ; they can all be reconciled in the fundamentally Darwinian idea of gradual change, due to the accumulation of small Men- delian mutations under the influence of Natural Selection.
The other main field in which fertile unification has taken place is that of development, though here the process has not yet gone quite so far.
A great difficulty for many people has been this. If Heredity, and So, in the long run Evolution, be based upon the collection of independent units which Mendelism reveals, how are we to account for the co-ordination so manifest in the organism at any and every stage, the correlated changes seen alike in individual develop- ment and racial evolution ? How, in fact, get an organic whole out of a mosaic, a unity out of a collection of units ?
The first and most general answer is that the problem is not one 'for biology alone ; the question asks itself in every field. How do the revolving electrons give us specifically different atoms according simply, to their number and arrangement ? How out of separate unit7 atoms do we get unitary substances with their own pro- perties, as water in place of bits of oxygen and hydrogen ? How, out of separate individuals, do we achieve a society ? Throughout Nature the unity of wholes is, somehow or other, achieved out of the separateness of unit parts, and I for one do not feel that any metaphysical quality of Emergence is needed. to account for the fact : but in each case the methods for achieving the coordinated whole arc different.
In our biological wholes, the chief methods are the follioWing. There is first the long -known method of nervous co-ordination ; "this, however, is the-last to come into play during development. Secondly, there is the effect of fiictiOn, of use and disuse. It is well known that many tissues, like muscle, grow stronger with .use ; and it has recently been established beyond doubt that not only the quantity but also the direction of tendon. fibres and bone-struts is determined by the stresses and strains to which they are subjected. Thus all the minutiae of adjustment of bones, tendons, muscles and other organs to their job within the body do not have to be fixed by heredity at all, but are dealt with ad hoc in each individual as it grows up. Old problems, such as the impossibility of imagining how all the eolOrdinated adaptations, of muscles, bones, sinews and blood-vessels, needed to support a large-horned deer's head, turn out to be 'no 'problems at all. Or, at least, they are not evolu- tionary problems, but, problems of development;. and the number of separate difficUlties is Much reduced -
Then there is the method of co-ordination by hormones' or chemical messengers discharged into the blood. By this means, not only can widely distant parts of the body be affected by a single agency, but they can all be affected simultaneously, which is of the utmost portance for orderly development. A good example is seen in the metamorphosis of tadpole into frog, which happens automatically when the amount of thyroid gland hormone in the blood reaches a certain level.
But this method can only work when the blood-stream is already there ; for earlier stages we used something else. And here various converging lines of research are introducing us to the notion of what for lack of further knowledge we call a morphogenetic or " form-producing " field. How it determines form is yet unknown ; the im-
portant fact for our present purpose is that it does so and that it is a field—in other words an organization in
which the forces at work are interconnected and have a definite configuration as a unitary whole. When you put a magnet under a piece of paper on which are strewn iron filings, you expose them to its magnetic field : as you move the magnet about, the form of the field remains the same, but different filings now constitute that form.
A bar-magnet has a field with two poles ; cut it in two, and each of the bits still has a field of the same general form.
So with early development and with regeneration. It takes place within the framework of a field of sorts. As with the bar-magnet, this kind of field can sometimes be subdivided cut a sea-urchin's leg in two, and each half still has a field of the same general type as the whole. I will give two other examples. Spemann has found that
the most rapidly growing region of the developing newt's egg has the capacity, if grafted into the. flank or belly of
another developing egg, of inducing there a second set of organs, a supernumerary embryo. Like a magnet, but in some as, yet not understood way, it induces a form-pro- ducing field in the more or less indifferent materials of the egg, and forces them to grow as it dictates. The per- sistence of this field throughout life has been demon- strated in adult newts; if you cut off the tail of a newt, wait till the first sign of a regenerating bud occurs, and then cut off a leg and graft the tail-bud on to the leg- stump, the material, though produced by the tail, is still indifferent, and is transformed by its new position in the form-field, so that it grows into leg !
Thus the mosaic of the hereditary units is forced to operate within the framework of a unitary field ; they can only come into play in accordance with its organization, and it has the quality of Wholeness from the start.
Finally, evidence is accumulating on all hands to show that many, perhaps most, Mendelian units work by controlling the ride of some process in the developing egg or embryo. This conception is likely to prove as fruitful as have corresponding conceptions in physical chemistry ; but it would take too long to go into detail of how and why.
The upshot is that, although in detail the relation of hereditary unit to finished character, of chromosome- outfit to adult organism, remains one of the most difficult and unexplored regions of biology, yet the country has becoMe opened up so that we are beginning to see its broad lines. The randomly assorted medicine cupboard of genes is entrapped from the start in an environment of order. They must act upon processes marching along in a given direetiOn ; they must 'operate within an organized field ; and, later, must express theMselves through the co-ordinating , agencies Of function, blood and nerves. Heredity has been the great unifier of biological theory during the past twenty , years ; the emphasis is now passing to Developinent. -