meditate on time and the mystery of the creative passage of nature without an overwhelming emotion at the limitations of human intelligence."

I place this saying over against the well-known words of Newton because the two utterances illustrate very well the change in our attitude towards what is meant by scientific discovery. But I must make it clear what I mean by the "passage of nature" because this is a notion far less subtle than that indicated by Mr. Whitehead. And I think that a candid and impartial survey of the speculative biology of the late nineteenth and twentieth centuries must force one to the recognition of a twofold passage of nature. Perhaps this is indicated even in Huxley's contrast of the cosmic and ethical processes but it is expressed, with the utmost clearness, in Bergson's vital impetus as opposed to the tendency of matter to pass into the inert condition. I take it that the fundamental concept of physical science is the second law of energetics-the universal augmentation of entropy-and I assume (though it is difficult to be sure) that nothing in the most modern results of mathematical relativity tends, in the least, to weaken this great conception. Nature, then, has direction, or passage, which is such that all that we recognize as physical change tends contiually to diminution: the Universe, regarded as a physical mechanism is one that is running down, or in Bergson's term, detending. This is the one aspect of the passage of nature.

To the biologist, however, it can only be one aspect. I am well aware that the entropy-increase law is a statistical one and that it can only hold true for organic entities which are above certain limiting magnitudes: for Maxwell's demons the law would have a double sign and the entropy of an isolated system would increase or decrease with equal probability. The biologist must recognize that, even in organic systems, entropy tends always towards augmentation but surely he misunderstands the meanings of reproduction and adaptation if he does not see that what he calls life is the incessant attempt of certain

physico-chemical systems that we call organisms to resist the increase of entropy. There is, therefore, a passage of nature which is not that tending to inert-materiality (that is, to statistical inertia) but which is the opposite to this passage and is what one must understand by life in the physical sense. I am not sure whether we ought to insist on this two-fold passage of nature or, perhaps, regard it as a double aspect, in some way or other, of the same condition. Is life something that resists the passage of inorganic nature, or is inorganic nature already inert and extended, while living systems pass through it? Perhaps one inclines to think about a two-fold passage because of some mental constraint that tends always to a dualism of one kind or another.

I suppose that biologists must accept the main result of generalized relativity: "the differentiation of the one quality of extension into time and space," but I confess that it is very difficult to do so. It seems to me that for speculative physiology space-time cannot be completely isotropic-the x, y, z and t-dimensions cannot be of the same quality. I take it that our notion of space rests entirely on our degrees of freedom of bodily mobility. I can move backwards and forwards, and from side to side with equal facility but not nearly so easily can I move upwards and downwards. And the equal freedom of mobility in the x, y-plane is only possible because I can turn my body round a vertical (≈)-axis in one direction or the other with equal facility but even then the turning movement from left to right is not quite the same as that from right to left but differs in some subtle way. And, of course, the difficulty of generating the z-dimension depends on the condition that our freedom of mobility is restricted because we move in a gravitational field. Only since we have become enabled to dispense (in thought) with the gravitational field as something physically unique has our space become truly isotropic.

Such as we are, however, the space-dimensions are not entirely isotropic and far less so is the t-dimension when

F 2

compared with the x, y, z-ones. The quality of duration I take to be entirely different from the others and we must, I think, regard it, with Bergson, as the cumulative continuity of life. It is a passage as well as the persistence of that which, in a sense, has passed. It is life-extension but it does not seem to me to be capable of "extensive abstraction" in Mr. Whitehead's The passage is not a uniform one (though I confess I find it difficult to say exactly what is meant by uniformity in a durational passage).*

sense.

Obviously we do not obtain the conception of a moment of organic duration by the method of extensive abstraction, for this "moment" depends on what Bergson calls the "rhythm of duration": thus the "event-particle" in the conscious life of a boy of fifteen is not the same as in the man of fifty, nor does it appear to be the same in the ephemeral insect as it is in the long-lived reptile. The matter, however, is much too difficult to be pursued here.

It is also necessary that I should deal with the purely biological conception of variability. In general we mean by a variation a deviation from a morphological type, but I generalize the notion so as to include also deviations of functioning, acting, response and mentality, perceiving no essential differences between these organic activities. The organic" type," whether it be that of form, or behaviour, or mentality is, of course, only a convenient abstraction, but the general notion of "types" has brought with it the conception of variability. It is very convenient, in our description of nature, to speak of specific types and then of variations or departures from them. Observations and experiments, we say, "ought" to give unique values but for the accompanying errors of methods. So, also,

* "Time" in the sense of life-extension I regard as "humped" in the neighbourhood" of a conscious entity in somewhat the same way as Mr. Eddington regards space as being "humped" or "peaked" in the neighbourhood of a material particle.

we postulate organic types which are accompanied by variations in the same way that experimental results are attended by error. Then we search (rather unsuccessfully, it must be admitted) for the "cause" of variability. Obviously, this conception of variability is the consequence of our adoption of the logical category of determinism.

Now there are organic activities that have all the appearance of spontaneity-whether these are truly spontaneous or not I do not argue-but there are also many activities which we call responses to events that occur "in the environment." These we can investigate and we can endeavour to establish a relation of functionality, in the mathematical sense, between the environmental "stimulus" and the organic response. would be easy, I think, to make a series of such responses, beginning with tropistic ones, passing through reflexes in the decerebrate animal, reflexes in the intact one, and ending with those responses which we call "intelligent."

It

In the various terms of such a series there will be "more or less" determinism, if one may say so without being misunderstood. At the one extreme we find (as in a tropism, or taxis) a degree of functionality which approximates closely to the behaviour of a compass needle in a variable magnetic field, and at the other we find that apparently capricious behaviour or functioning which must be so annoying to physiologists. I suppose that there must either be determinism, or no determinism, and so I have simply to reject the validity of this concept (except as a working method, of course) even at the risk of being exposed to the dreadful accusation of throwing overboard scientific method altogether!

One other thing I must endeavour to make clear-the distinction between the organic variations that are called "fluctuations" by biologists and those others that are called "mutations." The former are individual and acquired while the latter are congenital and are transmitted by hereditythey are not acquired. In the lower animals fluctuations, or

acquirements, do not materially influence the process of transformism and what are of significance from that standpoint are mutations, which do lead to transformism. In man, of course, certain fluctuations persist by reason of tradition : they are not bodily variations but means of action by tools (using the term "tool" in its most general significance). Now the distinction between fluctuations and mutations is evidently one that depends upon our distinction between a racial and individual life-passage. Life is, of course, a continuous career in the morphological sense: what is discontinuous in it is the personal passage which is marked by memory, blame, merit and responsibility—sin, if you like. The mutational variation belongs then to the racial passage and it is an acquirement of this continuous life-career; the fluctuating variation arises in the discontinuous personal life-passage, or career.

I return now, after this digression, to the saying of Newton. The ocean of undiscovered knowledge must, to him, have been like the material oceans explored by the voyagers of his century they were unknown but whatever was there did not depend, in itself, in the least upon the vessels and instruments of navigation; it was only revealed by those methods. So, to Newton, physical laws were there waiting to be discovered, so to speak, but even if they were to remain undiscovered they would still be there. Without doubt he could have made most of the discoveries of the eighteenth century and perhaps those of the nineteenth up to the time of Clerk Maxwell had he been capable for a long period of that sustained intension of mind of which he spoke, for (I take it) those discoveries were implicit in the creative work of his early lifetime.

But were the later physical and biological results of the later nineteenth and the twentieth century there in the same sense as were, for instance, planetary theory and tidal dynamics? Were the quantum hypothesis of radiation and