to swift charged particles which are bent in a magnetic field like H-atoms set in swift motion by a-particles, and which, indeed, are undoubtedly H-atoms. Since this effect is not observed in dry oxygen or carbon dioxide, it appears likely that some of the nuclei of nitrogen have been disintegrated by the action of the a-particles. Recently these experiments have been repeated by Mr Chadwick and myself under much better optical conditions for counting these comparatively weak scintillations. It has been found that, using radium C as a source of a-rays, the maximum range of the H-atoms from nitrogen atoms corresponds to 40 cm. of air, while the maximum range of the H-atoms from hydrogen, or any combination of hydrogen, is only 29 cm. under similar conditions. This result negatives the possibility that the presence of these H-atoms can be ascribed to any hydrogen contamination in the ordinary chemical sense

This observation opened up a simple method of examining other elements besides nitrogen. Experiments were made beyond the maximum range (29 cm.) of ordinary H-atoms, so as to be quite independent of the presence of free or combined hydrogen in the material under examination. In this way it has been found that similar particles are produced in boron, fluorine, sodium, aluminium, and phosphorus. No definite effect has so far been observed for other elements of the production of particles with ranges greater than 29 cm. of air. The question of the production of slower velocity H-atoms has not so far been examined. The range of penetration of the atoms from aluminium is specially marked, being more than 80 cm. While no definite proof has yet been obtained of the nature of these ejected particles, it seems probable that they are in all cases H-atoms liberated from the nuclei of the elements in question. It is of special interest to note that H-atoms are only liberated in elements whose mass is given by 4n+ 2 or 4n+ 3 where n is a whole number. No H-atoms are observed from elements like carbon, oxygen, and sulphur, whose mass is given by 4n. This is an indication that the a-particles are unable to liberate H-atoms from elements composed entirely of helium nuclei, but are able to do so from some elements composed of H-atoms as units as well as helium nuclei. It would appear as if the H-atoms were satellites of the main nuclear system and that one of them gained

sufficient energy from a collision with an a-particle to escape from its orbit with a high speed. If the long-range particles from aluminium are H-atoms, it can be calculated that the maximum energy of motion is somewhat greater than that of the incident a-particle, indicating that the escaping fragment of the atom has gained energy from the system. It is of special interest to note that, in the case of aluminium, the direction of escape of the H-atom is to some extent independent of the direction of the a-particle. Nearly as many are shot in the backward as in the forward direction, but in the former case the average velocity is somewhat smaller. No element of mass greater than phosphorus (31) has been found to yield H-atoms. It would appear as if the constitution of the nucleus undergoes some marked change at this stage.

It should be remarked that the disintegration observed in these experiments is on a very minute scale. Only about one a-particle in a million is able to get close enough to a nucleus to effect its disintegration.

So far we have only been able to observe those fragments of atoms which escape with sufficient speed to travel further than the a-particles. Another very important method of examining the effects produced within the range of the a-particle has been recently examined by Mr Shimizu. This depends on the discovery of Mr C. T. R. Wilson that the tracks of ionising radiations can be made visible by sudden expansion of a moist gas, so that each ion becomes the centre of a visible globule of water. Wilson had previously observed an occasional bend in the track of an a-particle, with a short spur attached, indicating the collision of an a-particle with an oxygen or nitrogen nucleus. By taking a large number of photographs of tracks of a-particles, Mr Shimizu found a number of cases in which the track of the a-particle near the end of its range showed two nearly equal forks. It can readily be shewn from the range and angle between the forks that these effects cannot be ascribed to a collision of the a-particle with a H-atom, or with a nucleus of hydrogen or nitrogen. It would appear not unlikely that these forks indicate an actual disruption of the atom in which a helium nucleus is released. While this conclusion is only tentative, it will be of great interest to follow up further this new method of attack of

a fundamental problem. It is remarkable that while only one a-particle in a million is able to liberate a H-atom from nitrogen, about one a-particle in 300 appears to show a forked track, indicating that this type of disintegration occurs much more frequently than the liberation of a Н-atom.

If this interpretation proves to be correct, it shows that the amount of energy required to liberate a helium nucleus from a complex nucleus of a light atom is not great. Such a result is not inconsistent with modern ideas of the relation between mass and energy, for the fact that the atomic masses of carbon and oxygen are very nearly integral multiples of the mass of the helium atom is an indication that the helium nuclei are bound loosely together. On the other hand, if we suppose the helium nucleus itself to be composed of four hydrogen nuclei and two electrons, the loss of mass in the structure indicates that the helium nucleus is so stable a structure that it should not be dissociated by even the swiftest a-particle. This conclusion is supported by experimental observations as far as they have gone.

III. THE THEORY OF EVOLUTION:
ARISTOTLE

PERHAPS an earlier subject of thought with both races and individuals than either the structure of the universe or that of matter is the origin and development of living creatures. There are stories of animal creation in every mythology; Sir J. G. Frazer writes that among primitive peoples they are of two kinds. Tillers of the soil, accustomed to the yearly flowering of fertile Earth, hold that the Creator modelled man and the animals from Earth's clay, and filled them with the breath of life. But pastoral peoples and hunters, who feel the close relationship of human and animal bodies, believe that one form of life changed into another, animals into other animals, or animals into men, so that tribes claim descent, protection, and patronymic from raven, salmon, or beaver ancestors. In historic times, these two patterns of thought alternately swayed the minds of men, till the scale was finally weighted by Darwin against the special creation theory, on the side of evolution, or the origin of higher species of living beings by descent from lower species.

Apart from the early myths, the philosophies of the Greeks are the first attempts to grapple with the problem. They are speculations only, but they tend in several cases to regard Nature as the result of a gradual development still in progress. Aristotle had some conception of a genetic series from lower to higher forms of life, and discusses the possible origin of animals either from an egg or a grub, concluding that the latter is more probable, since eggs always derive from some animal, while grubs are sometimes, as he erroneously thought, spontaneously produced. His theories have claim to remembrance, since he was a competent and thorough biologist, who observed, dissected, and described many species of animals.

HISTORIA ANIMALIUM

Book VIII. 1. 588 a. 1. (Oxford translation, 1910.)

We have now discussed the physical characteristics of animals and their modes of generation. Their habits and their modes of living vary according to their character and their food.

In the great majority of animals there are traces of psychical qualities or attitudes, which qualities are more markedly differentiated in the case of human beings. For just as we pointed out resemblances in the physical organs, so in a number of animals we observe gentleness or fierceness, mildness or cross temper, courage or timidity, fear or confidence, high spirit or low cunning, and, with regard to intelligence, something equivalent to sagacity. Some of these qualities in man, as compared with the corresponding qualities in animals, differ only quantitatively: that is to say, a man has more or less of this quality, and an animal has more or less of some other; other qualities in man are represented by analogous and not identical qualities: for instance, just as in man we find knowledge, wisdom and sagacity, so in certain animals there exists some other natural potentiality akin to these. The truth of this statement will be the more clearly apprehended if we have regard to the phenomena of childhood: for in children may be observed the traces and seeds of what will one day be settled psychological habits, though psychologically a child hardly differs for the time being from an animal; so that one is quite justified in saying that, as regards men and animals, certain psychical qualities are identical with one another, while others resemble, and others are analogous to, each other.

Nature proceeds little by little from things lifeless to animal life in such a way that it is impossible to determine the exact line of demarcation, nor on which side thereof an intermediate form should lie. Thus, next after lifeless things in the upward scale comes the plant, and of plants one will differ from another as to its amount of apparent vitality; and, in a word, the whole genus of plants, whilst it is devoid of life as compared with an animal, is endowed with life as compared with other corporeal entities. Indeed, as we have just remarked, there is observed in plants a continuous scale of ascent towards the animal. So, in the sea, there are certain objects concerning which one would be at a loss to determine whether they are animal or vegetable. For instance, certain of these objects are fairly rooted, and in several cases perish if detached; thus the pinna is rooted to a particular spot, and the solen (or razorshell) cannot survive withdrawal from its burrow. Indeed, broadly speaking, the entire genus of testaceans have a resemblance to vegetables, if they be contrasted with such animals as are capable of progression.

In regard to sensibility, some animals give no indication whatsoever of it, whilst others indicate it but indistinctly. Further, the substance of some of these intermediate creatures is fleshlike, as is the case with the so-called tethya (or ascidians) and the acalephae (or sea-anemones); but the sponge is in every respect like a vegetable. And so throughout the entire animal scale there is a graduated differentiation in amount of vitality and in capacity for motion.

A similar statement holds good with regard to habits of life. Thus of plants that spring from seed the one function seems to be the reproduction of their own particular species, and the sphere of action with certain animals is similarly limited. The faculty of reproduction, then, is common to all alike....Some animals, like plants, simply procreate their own species at definite seasons; other animals busy themselves also in procuring food for their young, and after they are reared quit them and have no further dealings with them; other animals are more intelligent and endowed with memory, and they live with their offspring for a longer period, and on a more social footing.