factor or gene may have manifold effects and that one character may be the product of many genes, yet the general idea remains that " the germ-plasm is made up of units that are independent of each other in at least two respects, namely, in that each one may change (mutate) without the others changing, and in segregation and in crossing over each pair is separable from the others " (Morgan, 1019, p. 240). The balance of present opinion seems to be strongly in favour of interpreting all inheritance as Mendelian, but it seems to many not unscientific to say that there is another mode of inheritance which may be described as " blending." There are indeed ways of explaining away blending as due to the cooperation of multiple Mendelian factors, and we are also now being introduced to " Mendelian characters which slowly blend "; but in an article which attempts a general survey it seems just to cite the considered opinion of an experi- menter and thinker like Prof. W. E. Castle (1916, p. 218), who writes: " It seems best, accordingly, to attempt neither with Galton to generalize all inheritance as blending nor with Johann- sen to treat all inheritance as alternative, but frankly to recog- nize the existence of two categories of cases distinct in their inheritance behaviour."
Inbreeding and OM/ftree Attention has often been directed to the evolutionary importance of inbreeding and outbreeding (exogamy and endogamy) , but it is only within recent years that firm foundations have been laid (see East and Jones, 1920). Professional breeders have recognized for a long time that close inbreeding, accompanied by the usual judicious selection and elimination, fixes desirable characters and leads toward a uniform and stable herd; and yet that it has disadvantages, being apt to lead to reduction in vigour, resisting power, fecundity, and even size. Till recently there has been no secure answer to the important question, whether the disadvantageous consequences are actually induced by the close breeding as such, or are simply brought to light and accentuated by it. Experiment has furnished the answer. It has been proved that the close inbreeding of fine stock, associated with the usual selection and elimination, may be persisted in for several generations without any unde- sirable consequences. Many fine breeds of animals have had very close inbreeding at their beginning; and there seems to be habitual endogamy among bees and ants. In certain forms of " isolation " there is bound to be close inbreeding. Further- more, it has been shown that the direct result of persistent inbreeding is to segregate within the stock a number of true breeding strains of similar individuals. If there were, at a given time, in a herd, say, four distinct hereditary factors relative to a particular character, such as the colour of the pellage, then if the factors illustrated Mendelian inheritance, the automatic effect of inbreeding would be to segregate four types, pure as regards the particular character in question. But some of the characters which thus become isolated may be undesirable " recessives," seldom seen under ordinary circumstances because they are hidden by their "dominant" counteractives. Thus the unde- sirable character of albinism, likely to be kept out of expression in conditions of exogamy, is isolated and brought to light in endogamy. These exposed recessives have given inbreeding its bad reputation, but now that the occurrence is understood the elimination of these recessives by the practical breeder becomes a more hopeful task. When the same undesirable qualities occur on both sides of the house, inbreeding tends to their diffusion and exaggeration, yet the result of modern experiment- ing is clear: " Inbreeding is not in itself harmful; whatever effect it may have is due wholly to the inheritance received " (East and Jones, 1920). But there is another phenomenon, namely the increased " vigour " which often rewards out- breeding. Darwin was strongly of opinion that the gain in constitutional vigour derived from an occasional crossing was a more important biological fact than the loss that often followed close inbreeding, and modern experimenters have confirmed his shrewd judgment. The outbreeding often has advantageous results like those that reward a notable improvement in nurture. There may be an increase in " vigour," resisting power, size, and tither good qualities. The reason for this frequently observed
" hybrid vigour " is probably to be found in the pooling of diverse hereditary resources of good quality, not in some vague physiological stimulus to the offspring. The general aspects of this kind of inquiry are very important. Thus one of the great trends of organic evolution has been toward the securing of cross-fertilization, though the range of crossing in different cases varies within wide limits. The survival value of this cross- fertilization probably lies in the fact that it promotes variability. It brings about a greater variety of raw material on which selective agencies can work. Similarly, for the wider ranges of cross-fertilization which we call outbreeding, the suggestion arises that this is fundamentally valuable in promoting varia- bility, both in the way of new patterns and fresh vigour. A new light is shed on the process of evolution if we consider the probability that success has rewarded the alternation of periods of exogamy with periods of endogamy. Thus there is a return to the position of Romanes, Gulick, and others, that " isolation " may count for much in evolution. Another ray of light, and a very welcome one, touches the dark problem of sterility. It is suggested by East and Jones that there are two quite different kinds of sterility of diverse origin: (i) Inbreeding tends to sort out homogeneous pure strains and in this sifting out the ability to reproduce may be lost; (2) outbreeding may bring together two germ-cells which are too incompatible, e.g. in their chromo- somal equipment, to allow of a continuance of the process of germ-cell making. Thus, the number of chromosomes in the two parents (e.g. horse and ass) may be too discrepant.
Nature or Nurture. There is a mounting up of evidence as to the influence of environment and function on the developing organism. Just as a certain quantity of food, oxygen, and mois- ture is essential if the inheritance is to be expressed, so is it with the wider environment of liberating stimuli, and so it is with functioning and exercise. Up to a certain point the chick's lung develops in virtue of the properties resident in the germinal material; beyond that point the development requires that the chick shall breathe. If the word " nurture," which Sir Francis Galton made technical, be used to include all manner of nutri- tional, environmental, and functional influences, it may be fairly said that modern zoology has been marked by an increasing appreciation of the importance of " nurture," as complementary, not antithetic, to the hereditary " nature." As Prof. T. H. Morgan says (1915):" every character is the realized result of the reaction of hereditary factors with each other and with their environment "; and again: " it is a commonplace that the en- vironment is essential to the development of any trait, and that traits may differ according to the environment in which they develop." Gudernatsch has shown that in tadpoles fed on thyroid there is differentiation without growth, while in tadpoles fed on thymus and spleen there is growth without differentiation. Fruit-flies (Drosophild) with a peculiar Mendelian abnormality may appear perfectly normal if reared in a dry bottle, but the presence within them of the " factor " for the abnormal charr acter may be demonstrated by rearing their offspring in a damp bottle (Morgan, 1915). Loeb has shown that it is easy to pro- duce a percentage of minnow (Fundulus) embryos with defective eyes by adding a very minute quantity of potassium cyanide to the water, or by exposing the developing eggs to low temperature: That is to say, relatively slight environmental changes may so alter the constitution of the developing embryo that a leap is taken in the direction of blindness. Peculiarities in nurtures may enhance or depreciate the hereditary virtues of the organ- isms; they may induce a modification which serves as a life- saving screen until an innate variation in the same direction has time to establish itself; in cases like mammals and seed- plants the nurtural condition of the maternal parent may in- fluence the vigour of the offspring during the ante-natal sym- biosis; changes in nurture, as Tower's experiments (1906, 1918) suggest, may be variational stimuli to the germ-cells.
In regard to the prolonged discussion over the transmissi- bility of exogenous somatic modifications, the chief need is still for more facts. It is admitted that somatic modifications may have secondary effects on the germ-cells and on the offspring
