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(especially in the case of the viviparous mammals), but this is not the same as the transmission of particular modifications as such or in any representative degree. It is probable, as Agar's experiments (1913) suggest, that long-continued, deeply satu- rating environmental and functional peculiarities may produce specific substances which enter into the cytoplasm of the germ- cell, or into the embryonic body, and may exert a very definite influence as long as they last; but there is as yet no convincing evidence that the resulting changes grip the constitution per- manently. J. T. Cunningham (1908) and others have suggested that particular modifications of an incisive sort may liberate very specific chemical substances, like hormones, and that these may be carried to the germ-cells and accumulate there with subsequent formative (morphogenic) effects. The number of competent zoologists who hold to some form of Lamarckism should be enough to restrain the impatient from attempting to close this important question or to dogmatize about it. As Prof. E. B. Wilson has wisely said: " In the present defective state of our knowledge we may well grant that there may be many a thing between germ-cell and body that is not yet dreamed of in our biological philosophy." It should be clearly under- stood that there is no depreciation of the varied importance of peculiarities in nurture on the part of those who are unable to find convincing evidence of the transmission of any exogenous somatic modification. It is incorrect to say that this scepticism leads to the view that individual experience counts for nothing racially. On the contrary, it is through individual experience that germinal variations or mutations are tried and tested.

Evolution Theory. It would be a contradiction in terms if the Evolution Theory did not evolve. What changes are in process? (i) Some methodological progress has been made in clarifying the concept of organic evolution (see Sorley, 1909). " Organic evolution is a continuous natural process of racial change in a definite direction, whereby distinctively new in- dividualities arise, take root, and flourish alongside of, or in place of, the originative stock" (Thomson, 1920, p. 378). We must also clearly distinguish this from (a) the " genesis " of a solar system or of a mountain or valley; (b) the "development" of an organism as an individual; and (c) the " history " of a human society. (2) Somewhat startling is the suggestion made by Bateson (1914), that we should consider " whether the course of evolution can at all reasonably be represented as an unpacking of an original complex, which contained within itself the whole range of diversity which living things present." " At first it may seem rank absurdity to suppose that the primordial form or forms of protoplasm could have contained complexity enough to pro- duce the divers types of life. But is it easier to imagine that these powers could have been conveyed by extrinsic additions?" In so far as this view means that there is nothing evolved which was not originally in kind involved, that there is nothing of lasting value in the end which was not represented in kind in the beginning, it may be philosophically tenable; but most biologists will probably adhere to the view that organic evolution has been a process of racial epigenesis (not an evolulio), with a frequent outcrop of genuine novelties, a synthetic complexifying by creative inventions and by trading with time, rather than an analytic simplification through the removal of inhibitions which allowed the original richness of endowment to express itself with increasing fullness. There was a time when there were no birds or mammals; they came into being, and they were new ideas; they made a new world. (3) While there is practical unanimity among the zoologists of to-day as to fact of organic evolution, there is a clearer apprehension than there was a genera- tion ago of the detailed difficulties. These difficulties are chiefly of three kinds: (a) that little is known with certainty in regard to the pedigree of the great phyla, such as Vertebrates, and as to the way in which the biggest steps were taken, like the passage from Protozoa to Metazoa; (b) that there naturally remains much uncertainty in regard to the factors operative in the historic process, and as to the conditions implied in the hereditary or ge- netic relation between one generation and another; and (c) that in regard to the originative factor termed for convenience " vari-

ability," our ignorance is still immense. Bateson expressed the mood of many when he said (1913, preface): "That species have come into existence by an evolutionary process no one seriously doubts; but few who are familiar with the facts that genetic research has revealed are now inclined to speculate as to the manner by which the process has been accomplished. Our knowledge of the nature and properties of living things is far too meagre to justify any such attempts." And later in the same work (p. 97), he says: " We have not even an inkling of the steps by which a Silver Wyandotte fowl descended from Callus bankiva, and we can scarcely even believe that it did." (4) It is pessimistic to say that we are no nearer an understanding of the central problem of aetiology, namely the origin of the new, i.e. of the variations or mutations which form the raw material of progress or the reverse. The ground has been cleared, though the problem remains unsolved, (a) If from the observed differ- ences between the members of the same species there can be subtracted all the peculiarities that are associated with age and sex, and likewise all that can be reasonably interpreted as in- dividually acquired exogenous somatic modifications (directly due to peculiarities of nurture, whether environmental, nutri- tional, or functional), then we get at the true variations and mutations inborn, not acquired endogenous, not exogenous blastogenic, not somatogenic expressions or outcomes, not indents or imprints. If there be no convincing evidence of the transmission of extrinsic modifications, as such, or in any repre- sentative degree, then they cannot be included, in the first instance at least, among the raw materials for evolution, which are thus confined to what is left when the modifications are subtracted from the total of observed differences. That some at least of this residue, as true variations, are very transmissible, is quite certain. This is the first clearing of the ground, (b) But within the large category of inborn changes it seems possible to distinguish minute continuous variations from brusque salta- tions (Darwin's " sports," Gallon's " transilient variations," Bateson's " discontinuous variations," and of course the " muta- tions " of De Vries; all these being practically the same, though investigated with ever-increasing care). The minute continu- ous variations, on the other hand, are of the nature of " a little more or a little less," and they show intergrades. Their trans- missibility has not been much studied, but has been proved in a few cases. The mutations, whether large or small, are more deserving of the title qualitative; they often show a measure of perfectness from their first appearance, they are without inter- grades, they are markedly transmissible, and they are demon- strably at the origin of some new varieties of domesticated animals and cultivated plants. They correspond to, or are im- plicated in, those " unit characters " of species or varieties which illustrate Mendelian inheritance in their recurrence from generation to generation. Now whether we attend to the con- tinuous or to the discontinuous variations, we can imagine them to arise in the course of the shufflings of the hereditary cards (the chromosomes and their microsomes) during the intricate processes of maturation and fertilization. The reduction or exaggeration of a quality, the dropping out of a character alto- gether, a " double dose " of a character, a rearranged pattern of hereditary items, may all be interpreted as due to permuta- tions or combinations of hereditary factors or genes during the intricate manoeuvres preceding the completion of the fertiliza- tion of the ovum. The evidence is very strong that the segrega- tion of the factors of Mendelian characters takes place in the reduction divisions of the germ-cells in the process of matura- tion. An interesting corroboration has been furnished by the recent experiments of Prof. W. E. Agar (1920) with a partheno- genetic " clone " (pure line) of a hybrid water-flea (Daphnia pertusa X Daphnia pulex). No reduction divisions occur in a parthenogenetic lineage, and no Mendelian variations occurred. Along with the rearrangements which may be effected in the maturation of the germ-cells rearrangements in which such phenomena of chromosomal behaviour as " crossing-over " must be taken account of another possibility of change is afforded at the beginning of each individual life; for there, in all