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ordinary cases, two very complex systems, of dual origin, become a new unity which normally develops into a harmonious organ- ism. Some modern evolutionists, such as Lotsy (1916), attach great importance to crossing as a cause of variations.

If it be said that such opportunities as have been mentioned could account for nothing more than rearrangements, pluses and minuses, gains and losses, but not for any distinctively novel change, the difficulty of interpretation increases. But even among Mendelians it is sometimes admitted that the factors or genes may themselves change. How this might happen is at present the subject of speculation, but this need not be unreasonable, and it is sometimes backed by the beginning of experiment. Three suggestions may be stated. First, it may be, as Weismann believed, that deeply saturating environmental influences act through the body as variational stimuli on the germ-cells, provoking them to change. It is known that changes in a cell's environment may cause disturbance of the process of cell-division, and this, as Bateson has pointed out, may be the fans el origo of germinal variation. Apart from demonstrable external stimuli, it is possible that the chromosomes of the germ-cells may change suddenly like mutating Bacteria, or may undergo age changes, or may exhibit periodic reorganizations, like the remarkable endomixis that occurs about once a month in a "pure line" of Paramoecium, or may show rejuvenescence- changes, like those which occur in some cases of regeneration and asexual multiplication. It must be borne in mind that the chromosomes are very complex living bodies, and that differ- ences between species are sometimes the expressions of visible differences in the chromosomes, as Gates (1915) has shown in the case of Oenothera.

The Mendelian conception of unit-characters, whose germinal factors or genes behave like discrete unities in inheritance, neither blending nor fragmenting, has had to be corrected by fuller knowledge. Thus Prof. T. H. Morgan (1919) lays emphasis on the conclusions: first, that each gene may have manifold effects on the organism; and second, that every part of the body, and even each particular character, is the product of many genes. The phenomenon of linkage also shows that chains of genes may keep together in inheritance instead of there being free assort- ment of the various links in the chain. Thus from the idea of extreme particulateness which Mendelism at first suggested, there is a return toward the Darwinian idea, on which Sir E. Ray Lankester, especially, has laid emphasis, of the correlation of variations. Several apparently independent changes may be diverse outcrops due to one disturbance. A change in some particular kind of metabolism may reverberate through the whole body. Another idea of considerable importance is that of tem- poral variations, that is to say, alterations in the " time " or rate or rhythm of certain metabolic processes, or in the duration of particular phases in the life-cycle. Many changes of great adaptiveness are interpretable as lengthenings out or shorten- ings down of particular chapters in the life-history. In the influence of internal secretions in backboned animals there is a known method whereby these variations in tempo might be brought about; and it is worthy of notice that in Sebright poultry, where the cocks are hen-feathered, an endocrine secre- tion depends on a genetic factor inherited in the same way as are other genetic factors (see T. H. Morgan, 1919, p. 243).

Many palaeontologists, with remarkable concurrence, have reached the conclusion that an evolving line often changes not by discontinuous steps (saltations or abrupt mutations), but by continuous minute steps. This is well illustrated by Dr. Rowe's study of the species of the sea-urchin Aficraster in the chalk of S.E. England and just as well by the evolution of Mammalian teeth. Speaking of Tertiary mammals, Dr. W. D. Matthew writes (1910): " The more complete the series of specimens, the more perfect the record in successive strata, and the nearer the hypothetic centre of dispersal, the closer do we come to a phyletic series whose intergrading stages are well within the limits of observed variation of the race."

Whether the individual changes in the past were continuous and minute transitions, like those which Dr. Duerden finds occur-

ring to-day in the ostrich, or were discontinuous and minute saltations, or both, no one can tell; and the question does not seem very important. Even in ordinary growth what seems by one mode of measurement continuous, may be shown by another and finer method to progress rhythmically, even to seeming fits and starts. But the palaeontologist tells us more; he can arrange his species in series in a time-succession. " There is not merely transition, but transition in orderly sequence, such as can be represented by a graphic curve of simple form " (Bather, 1920, p. 73). Moreover, in this seriation there may be gradual increase or decrease as in the number of spines or tubercles, or in the size of horns or digits; and this gives the palaeontologist an advantage over the neontologist, inasmuch as he can point to progression or retrogression extending throughout millenia. It is for the neontologist to assist the palaeontologist by point- ing to determinate variation occurring in generations of living animals; for it may have been by such mode of variability that the seriations of the rock record were produced (see Bather, 1920, p. 74).

There is a growing body of evidence in support of the view that variation is often a very definite kind of organic change. This cannot be assured from the movement of a species as such; for the inferred orthogenesis might be the outcome of selection, sifting out those best adapted to a particular environment which might also be changing in a definite direction, say of increased salinity or aridity. But when large numbers of specimens are procurable, and can be studied in successive years (or centuries in strata), the non-occurrence of variations off the line is signifi- cant. If this be so, it is not necessary to look for any particular impulse from within to account for the definiteness. It may be enough to recognize that the unified organization, stereochemical in part at least, must to a certain extent predetermine what variations (other than pathological disintegrations) emerge.

Here may also be mentioned the suggestion of dichotomous variation, as between types of passive and active (anaboh'c and katabolic) preponderance, and as resulting in contrasts of allied forms, like bee and wasp, moth and butterfly, sheep and goat, and to larger differences as of reptile and bird, even of plant and animal, female and male. (See P. Geddes, VARIATION, E.B., gth ed.; and Geddes and Thomson, Evolution, 1911.)

Natural Selection. The zoologist of to-day has to face the reconciliation of two sets of facts as regards natural selection. In a few cases, such as Weldon's crabs, it has been demonstrated in actual effective operation, and Cesnola's test of the protec- tiveness of colouring by tethering green mantises with silk threads on brown herbage and brown variants on green herbage, gave vividly convincing results. Working with " hooded rats," Prof. W. E. Castle selected simultaneously in two opposite directions; and produced one race black all over except for a white patch of variable size underneath, and another race white all over except for the top of the head and the back of the neck, which are black. On the other hand, it has been shown by Johannsen, de Vries, Jennings, Pearl and others that selection does not count for much within pure lines or inbred stocks. Within an inbred race of guinea-pigs, for instance, a ne plus ultra may be reached, beyond which no amount of selection is of any avail within the period of experimentation. Abundant " fluctua- tions " of a quantitative sort occur, but these cannot be used as a basis for selection since they are not transmissible. They are probably for the most part of the nature of somatic modifications.

The estimate of the scope of natural selection is plainly affected by the view taken in regard to the raw materials supplied. If a new position of structural or functional equilibrium is reached by mutational abruptness, there is no need in such a case to burden natural selection with the task of gradually accumulating minutiae. On the other hand, due attention should be given to de Vries's declaration (1909, p. 83): "Thus we see that the theory of the origin of species by means of natural selection is quite independent of the question, how the variations to be selected arise? They may arise slowly, from simple fluctuations, or suddenly, by mutations; in both cases natural selection will take hold of them, will multiply them if they are beneficial,