There is another occasion of an entirely different kind when the electron is greatly in evidence and displays effects which are most astonishing and significant. Every atom of radium or other radio-active substances sooner or later meets with the catastrophe in which its life as radium ends and atoms of other substances are formed. At that moment occurs the emission which is the characteristic property of the substance. One of the radiations emitted consists of high-velocity electrons, moving, some of them, nearly as fast as light.

Now it is found that when the speed approaches that of light, 186,000 miles or 3X 1010 centimetres per second, the energy is higher than it should be if it followed the usual rule, viz. energy is equal to half the mass multiplied by the square of the velocity. It would seem that an electron moving with the velocity of light would have infinite energy; or, to put the matter in another way, the experimenter in his laboratory can never hope to observe an electron moving so fast; it would be the end of his laboratory and of himself if ever it turned up.

Linked up with this result is the very strange fact that no one has ever been able to find any direct evidence of the existence of the ether, which is postulated in order to carry light-waves. It has been pictured as a medium through which the heavenly bodies move, and to which their motions may be referred. But when light is launched into the ether, its apparent velocity must depend on whether it travels with or against the drift of the ether through the laboratory where the measurement is made. The experiment has been performed without the discovery of any such difference, although the method was amply accurate enough to detect the effect that might be expected. It was afterwards shown that the negative result might be explained by supposing that a measure of length varied in length according to whether it was travelling with or against the ether. But the continual

failure of all such experiments has led to a remarkable hypothetical development with which the name of Einstein is firmly connected. It is supposed that some flaw must exist in our fundamental hypotheses, and that if this were corrected we should then find that we ought to get the same value for the velocity of light however and whenever we measured it, and at the same time we should find that no measurement of the velocity of a body moving relative to the observer would ever equal the velocity of light. The hypothesis denies the existence of an absolute standard to which motions can be referred, and insists that they must all be considered relatively to the observer. It is called the principle of relativity. Calculations of its consequences begin with the necessary changes in the fundamentals, such as Einstein has introduced.1

Time does not allow me to say more of the innumerable ways in which electrons play an essential part in all the processes in the world. We have long believed that this is so, but the picture has never been so clear to us as it is now; and with our understanding our power is increased. Yet once more the illumination of our understanding comes from our recognition that Nature has preferred the discrete to the continuous and that electricity is not infinitely divisible but is, like matter, and even more simply than matter, of an atomic structure. And we have found the unit and learnt how to handle it.

It is even more strange that it may now be said of energy that there are signs of atomicity. It may seem absurd to think that the energy which is transformed in

1 Since this address was given, the results of the Eclipse Expedition to Brazil are considered to have confirmed in a satisfactory manner one of the most remarkable deductions made by Einstein from the principles which he maintains. The matter has roused so much interest that some of the leading exponents of the relativity principle have published careful accounts intended for students not familiar with it: it would therefore be superfluous to discuss the matter here.

P 2

any operation is transformed in multiples of a universal unit or units, so that the operation cannot be arrested at any desired stage but only at definite intervals. Indeed we have no right to assert that this is always true. But undoubtedly there are cases in which the atomicity of energy is clear enough, as for example in the interchange of energy between electrons in motion and radiation. It is remarkable that when radiation sets an electron in motion, the electron acquires a perfectly definite speed depending only on the wave-length of the radiation and not on its intensity, and has apparently absorbed from the radiation a definite unit of energy. Radiation of a particular wave-length cannot spend its energy in this way except in multiples of a certain unit, because each of the electrons which it sets in motion has the same initial energy, which it must have got from the radiation. In other words, energy of radiation of the particular wave-length can only be transformed into energy of movement of electrons in multiples of a certain quantum' peculiar to that wave-length. The intensity of the radiation, that is to say, the amount of energy moving along the beam, can only affect the number of electrons set in motion and not the speed of any one of them. During the last few years a very extraordinary theory has been developed on the basis of these and similar facts. I doubt if it would be more profitable to give further instances at present, but I have mentioned it because it seems to show looming on the horizon of our knowledge another tendency of Nature to make use of the atomic principle.

I will only add that the whole position of physics is indeed at this time of extraordinary interest, and at any moment there may be some great discovery or illuminating thought which will explain the present startling difficulties and open up new worlds of thought.

FOR REFERENCE

Bragg, Rays and Crystals (Ball & Sons).

IX

PROGRESS IN BIOLOGY DURING THE

LAST SIXTY YEARS

PROFESSOR LEONARD DONCASTER, F.R.S.

ON November 24, 1859, The Origin of Species was published, and this date marks the beginning of an epoch in every branch of biology. Before it, Biology had been almost entirely a descriptive science, but within a few years after the publication of the Origin its effects began to colour all aspects of biological research. A coordinating and unifying principle had been found, and the leading idea of biologists ceased to be to describe living things as they are, and became transformed into the attempt to discover how they are related to one another. The first effect of this change of attitude was chiefly to turn biologists towards the task of tracing phylogenetic or evolutionary relationships between different groups of animals-the drawing up of probable or possible genealo gical trees and the explanation of natural classification on an evolutionary basis. When once, however, the notion of cause and effect, or more correctly of relationship, between the phenomena seen in living beings had become familiar to biologists, it spread far beyond the limits of tracing genealogical connexions between different animals and plants. It made possible the conception of a true Science of Life, in which every phenomenon seen in a living organism should fall into its true place in relation to the rest, and in which also the phenomena of life should be correlated with those discovered in the inorganic sciences of Chemistry and Physics.

The history of the various branches of biological science in the past sixty years reflects the general course of these tendencies. Until shortly after 1859, the study of morphology, or the comparative structure of animals (and of plants) was intimately related with that of physiology, that is, with the study of function. In the years following the appearance of the Origin, however, anatomists and morphologists were seized with a new interest. For the time at least, the chief aim in studying structure was no longer to explain function, but rather to explain how that structure had come into being in the course of evolution, and how it was related with homologous but different structures in other forms. The result was a tendency to a divorce between morphology and physiology, or at least between morphologists and physiologists, which led to the division into two more or less distinct sciences of what had hitherto been regarded as closely inter-related branches of one. The greater men of the early part of the period, such as Huxley, remained both morphologists and physiologists, but most of their followers fell inevitably into one or the other group, and in discussing the later phases of biological progress it will be necessary to keep them separate.

Apart from its effect on the systematic and anatomical side of Biology, the idea of Evolution, and especially of Darwin's theory of Natural Selection, had important consequences on that side of the science which may be described as Natural History. Before the appearance of Darwin's work, Natural History consisted chiefly in the observation and collection of facts about the habits and life-history of animals and plants, which as a rule had no unifying principle unless they were used, as in the Bridgewater Treatises, to illustrate the power, wisdom, and goodness of God'. Now, however, a new motive was provided that of discovering the uses to the organism of