按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
he same character occurs also in some of the seedlings； but anything approaching a constant race has not been produced。
Another means of producing new races has been attempted by Blaringhem。 （Blaringhem； 〃Mutation et Traumatisme〃； Paris； 1907。） On removing at an early stage the main shoots of different plants he observed various abnormalities in the newly formed basal shoots。 From the seeds of such plants he obtained races； a large percentage of which exhibited these abnormalities。 Starting from a male Maize plant with a fasciated inflorescence； on which a proportion of the flowers had become male； a new race was bred in which hermaphrodite flowers were frequently produced。 In the same way Blaringhem obtained； among other similar results； a race of barley with branched ears。 These races； however； behaved in essentials like those which have been demonstrated by de Vries to be inconstant； e。g。 Trifolium pratense quinquefolium and others。 The abnormality appears in a proportion of the individuals and only under very special conditions。 It must be remembered too that Blaringhem worked with old cultivated plants； which from the first had been disposed to split into a great variety of races。 It is possible； but difficult to prove； that injury contributed to this result。
A third method has been adopted by MacDougal （MacDougal； 〃Heredity and Origin of species〃； 〃Monist〃； 1906； 〃Report of department of botanical research〃； 〃Fifth Year…book of the Carnegie Institution of Washington〃； page 119； 1907。） who injected strong （10 percent） sugar solution or weak solutions of calcium nitrate and zinc sulphate into young carpels of different plants。 From the seeds of a plant of Raimannia odorata the carpels of which had been thus treated he obtained several plants distinguished from the parent…forms by the absence of hairs and by distinct forms of leaves。 Further examination showed that he had here to do with a new elementary species。 MacDougal also obtained a more or less distinct mutant of Oenothera biennis。 We cannot as yet form an opinion as to how far the effect is due to the wound or to the injection of fluid as such； or to its chemical properties。 This； however； is not so essential as to decide whether the mutant stands in any relation to the influence of external factors。 It is at any rate very important that this kind of investigation should be carried further。
If it could be shown that new and inherited races were obtained by MacDougal's method； it would be safe to conclude that the same end might be gained by altering the conditions of the food…stuff conducted to the sexual cells。 New races or elementary species； however； arise without wounding or injection。 This at once raises the much discussed question； how far garden…cultivation has led to the creation of new races？ Contrary to the opinion expressed by Darwin and others； de Vries （〃Mutationstheorie〃； Vol。 I。 pages 412 et seq。） tried to show that garden…races have been produced only from spontaneous types which occur in a wild state or from sub…races； which the breeder has accidentally discovered but not originated。 In a small number of cases only has de Vries adduced definite proof。 On the other side we have the work of Korschinsky （Korschinsky； 〃Heterogenesis und Evolution〃； 〃Flora〃； 1901。） which shows that whole series of garden…races have made their appearance only after years of cultivation。 In the majority of races we are entirely ignorant of their origin。
It is； however； a fact that if a plant is removed from natural conditions into cultivation； a well…marked variation occurs。 The well…known plant… breeder L。 de Vilmorin （L。 de Vilmorin； 〃Notices sur l'amelioration des plantes〃； Paris； 1886； page 36。）； speaking from his own experience； states that a plant is induced to 〃affoler；〃 that is to exhibit all possible variations from which the breeder may make a further selection only after cultivation for several generations。 The effect of cultivation was particularly striking in Veronica chamaedrys （Klebs； 〃Kunstliche Metamorphosen〃； Stuttgart； 1906； page 152。） which； in spite of its wide distribution in nature； varies very little。 After a few years of cultivation this 〃good〃 and constant species becomes highly variable。 The specimens on which the experiments were made were three modified inflorescence cuttings； the parent…plants of which certainly exhibited no striking abnormalities。 In a short time many hitherto latent potentialities became apparent； so that characters； never previously observed； or at least very rarely； were exhibited； such as scattered leaf… arrangement； torsion； terminal or branched inflorescences； the conversion of the inflorescence into foliage…shoots； every conceivable alteration in the colour of flowers； the assumption of a green colour by parts of the flowers； the proliferation of flowers。
All this points to some disturbance in the species resulting from methods of cultivation。 It has； however； not yet been possible to produce constant races with any one of these modified characters。 But variations appeared among the seedlings； some of which； e。g。 yellow variegation； were not inheritable； while others have proved constant。 This holds good； so far as we know at present； for a small rose…coloured form which is to be reckoned as a mutation。 Thus the prospect of producing new races by cultivation appears to be full of promise。
So long as the view is held that good nourishment； i。e。 a plentiful supply of water and salts； constitutes the essential characteristic of garden… cultivation； we can hardly conceive that new mutations can be thus produced。 But perhaps the view here put forward in regard to the production of form throws new light on this puzzling problem。
Good manuring is in the highest degree favourable to vegetative growth； but is in no way equally favourable to the formation of flowers。 The constantly repeated expression； good or favourable nourishment； is not only vague but misleading； because circumstances favourable to growth differ from those which promote reproduction； for the production of every form there are certain favourable conditions of nourishment； which may be defined for each species。 Experience shows that； within definite and often very wide limits； it does not depend upon the ABSOLUTE AMOUNT of the various food substances； but upon their respective degrees of concentration。 As we have already stated； the production of flowers follows a relative increase in the amount of carbohydrates formed in the presence of light； as compared with the inorganic salts on which the formation of albuminous substances depends。 （Klebs； 〃Kunstliche Metamorphosen〃； page 117。） The various modifications of flowers are due to the fact that a relatively too strong solution of salts is supplied to the rudiments of these organs。 As a general rule every plant form depends upon a certain relation between the different chemical substances in the cells and is modified by an alteration of that relation。
During long cultivation under conditions which vary in very different degrees； such as moisture； the amount of salts； light intensity； temperature； oxygen； it is possible that sudden and special disturbances in the relations of the cell substances have a directive influence on the inner organisation of the sexual cells； so that not only inconstant but also constant varieties will be formed。
Definite proof in support of this view has not yet been furnished； and we must admit that the question as to the cause of heredity remains； fundamentally； as far from solution as it was in Darwin's time。 As the result of the work of many investigators； particularly de Vries； the problem is constantly becoming clearer and more definite。 The penetration into this most difficult and therefore most interesting problem of life and the creation by experiment of new races or elementary species are no longer beyond the region of possibility。
XIV。 EXPERIMENTAL STUDY OF THE INFLUENCE OF ENVIRONMENT ON ANIMALS。
By JACQUES LOEB； M。D。 Professor of Physiology in the University of California。
I。 INTRODUCTORY REMARKS。
What the biologist calls the natural environment of an animal is from a physical point of view a rather rigid combination of definite forces。 It is obvious that by a purposeful and systematic variation of these and by the application of other forces in the laboratory； results must be obtainable which do not appear in the natural environment。 This is the reasoning underlying the modern development of the study of the effects of environment upon animal life。 It was perhaps not the least important of Darwin's services to science that the boldness of his conceptions gave to the experimental biologist courage to enter upon the attempt of controlling at will the life…phenomena of animals； and of bringing about effects which cannot be expected in Nature。
The systematic physico…chemical analysis of the effect of outside forces upon the form and reactions of animals is also our only means of unravelling the mechanism of heredity beyond the scope of the Mendelian law。 The manner in which a germ…cell can force upon the adult certain characters will not be understood until we succeed in varying and controlling heredit