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WEB SITEZONE & GEARIN
kernels, white kernels, kernels of intermediate bluish-white tint and kernels streaked with blue and white. The streaked flowers and kernels of these two cases are due to mosaic inheritance.
Or the apparent dominance of the contrasting characteristics may be proved to have something of real dominance about it, as Miss McCracken has so clearly shown in her studies of the inheritance of dichromatism in the little beetles Lina lapponica (Fig. 117) and Gastroidea dis
similis. Here the Fig. 116.–At left an ear of fielel corn; at right an ear of
first two or three sweet corn; and in the middle a hybrid of these two, showing alternation of kernels resembling those of generations behave each different parent. (After de Vries.)
in true Mendelian
manner, but with successive generations the dominant character is plainly seen to be gradually extinguishing the recessive character in the crossbred groups, so that in the seventh generation after the original cross-mating the Mendelian ratio of 2 to 1 in the cross-bred group is changed to 28 to 1.
There may occur also a breaking up or decomposition of the parental varietal characters, which may mean that the dominant and recessive char- Fig. 117.—Lina lapponica, showing its two forms, acters are not simple one black and one spotted. (Aster McCracken.) ones but are complexi. e., really the resultant of several combined characteristics; or it may mean that there exists a real instability in the parent type and that the stimulus or influence of the cross-mating is all
that is needed to break down this weak apparent stability of the type and allow its component characters (the elementary units of de Vries) to recombine into various new and differing types. This condition seems to be that which results in the extraordinary variation so commonly observed by plant and animal breeders as brought out by hybridization, and which is constantly made use of by these breeders. Luther Burbank depends very largely on this initial abundant and eccentric variation induced by wide hybridizations for “starters” for his work of producing “new creations.”
So in accepting Mendel's laws of heredity we must bear clearly in mind that they by no means apply to all, or, at any rate, that our present knowledge of them does not include their application to all, cases and categories of inheritance.
INHERITANCE OF ACQUIRED CHARACTERS
Tout ce qui a été acquis, tracé ou changé dans l'organisation des individus, pendant le cours de leur vie, est conservé par la génération et transmis aux nouveaux individus qui proviennent de ceux qui ont eprouvé ces changements.-LAMARCK.
Heaven forfend me from Lamarck nonsense of a tendency to progression, adaptation from the slow willing of animals, etc.; but the conclusions I am led to are not wholly different from his, though the means of change are wholly so.—Darwin to HOOKER, 1848.
THE “fourth Law of Evolution," as expressed by Lamarck in his “Zoological Philosophy," reads as follows: "All that has been acquired, begun, or changed in the structure of individuals in their lifetime, is preserved in reproduction and transmitted to the new individuals which spring from those which have inherited the change.”
This principle was used by Lamarck as one of the fundamental elements in his theory of the transmutation of species. For nearly a hundred years it attracted little attention, being accepted as a part of the law of heredity by most persons, even by those most opposed to the essential part of Lamarck's theory, the derivation or transmutation of species. Among others, Darwin accepted it as one of the factors in evolution of forms. With Herbert Spencer it became one of the fundamental principles of the philosophy of Evolution. Mr. Spencer states the proposition in this way: “Change of function produces change of structure: it is a tenable hypothesis that changes of structure so produced are inherited.” For the supposed inheritance of characters produced by the impact of environment or by resultant activities of the individual the term progressive heredity has been devised. The fact of the existence of pro
gressive heredity, more or less taken for granted by writers of the last century, was flatly denied by Dr. August Weismann, who insisted that it was necessary that the theory of the inheritance of characters acquired in the lifetime of the individual should no longer be accepted without definite proof. In the theory of heredity through the development of the germ cell controlled by influences exerted by structures within the nucleus, Weismann found no room for the inheritance of characters not preestablished within this germ. External influences in general cannot reach the germ cells, and throughout nature the germ cells are elaborately protected from the direct influence of external conditions. This attack upon an ancient theory roused its supporters to defend their faith and to search for evidence to support it. A temporary division of naturalists into two schools arose as a result of this discussion. Those who held with Lamarck and Spencer that characters gained in the life time of the individual, and not received from ancestors possessing them, became hereditary, were known as Neo-Lamarckians. Those who, with Weismann, denied the existence of this factor and from a necessity, real or fancied, laid special stress on the Darwinian principle of natural selection, assumed the title of Neo-Darwinians. In their hands the Darwinian principle became the all-powerful factor in evolution, a theory of Allmacht which was soon questioned from other quarters and by those not considered as Neo-Lamarckians. Prominent among the leaders of the NeoLamarckians were Herbert Spencer, Haeckel, Nägeli, Cope, Eimer, Hyatt, Gadow, Dall, Packard, and others. Among the recognized Neo-Darwinians were Weismann, Wallace, Huxley, Gray, Brooks, Lankester, and others. After some years of controversy, mostly theoretical, the discussion has been tacitly dropped by biologists generally. It is recognized that the sole crucial test is that of experiment, that experiment is not easy, inasmuch as it is very difficult to show that any given trait in heredity really belongs to the category of acquired characters, and that in no case has it been indubitably shown that any character not inborn has been inherited. Moreover the studies of the germ cell and the physical basis of heredity tend to show that the structures of the germ cell are more complex and that the processes of heredity are in a sense more mechanical than could have been supposed in the time of Lamarck or even that of Darwin or Spencer. The characters shown by any adult individual are all in a sense acquired characters, their development dependent largely on nutrition and on the influences of environment. The facts of heredity show that it is not the actual traits of the parents, but rather their potentialities which are inherited. Moreover, acquired characters are simply matters of degree of development. They represent in no case anything qualitatively new. Taking the modern theories of heredity, it is perhaps not conceivable that “all that is acquired, begun, or changed " in the physical or mental life of the individual should produce a corresponding change in the germ cells, or in the cells from which these are thrown off. On the other hand, Dr. Weismann has admitted the possibility that one-celled animals and animals of simple structure in which the germ cell shares in the general relation of the body cells to the environment may be effected by developmental conditions. In other words, the inheritance of acquired characters may be a reality in the development of Protozoa, the simpler Metazoa, and the lower types of plants, but this condition does not obtain among the higher forms. In much of the discussion on this subject the term “acquired characters” is used with an uncertain or double meaning. The term should be limited to traits of the individual which were not inborn or blastogenic, and would not be exhibited in the natural or usual development of the individual. In general, such traits would arise either from the operation of use or disuse of parts, or other functional stimulation derived from the environment. An illustration of an acquired character resulting from use and disuse would be the increased size of the arm in the blacksmith, or the decreased leg muscles of the tailor. The training of a musician or of a mathematician would give increased power along the lines of the training. The neglect of the musical or of mathematical ability would lead to the relative mediocrity of this form of ability. Education in a general way increases mental capacity: neglect of education allows it to become relatively less. The supposed inheritance of results of civilization forms an important part of the philosophy of Herbert Spencer. Is civilization the inheritance of the power gained by past successes, or is it simply the acquisition of the machinery which past successes have produced? As to this Herbert Spencer