Professor Tower has experimented for a number of years with various species of Leptinotarsa, the potato beetle. By varying the conditions of temperature, humidity and atmospheric pressure when females were laying their eggs, he reports having produced variations in the young which came from these eggs although the mothers themselves were not changed. According to Professor Tower slight increase or decrease in these environmental factors stimulated the activity of the color producing ferments, giving rise to melanic or darker individuals. Greater increase or decrease, inhibited them and produced albinos. He found also that at times the same stimulus might show different results in different eggs. The effect, therefore, is a general and not a specific one. Ordinarily the eggs of these beetles are laid in batches. When one of these batches was laid and left under normal conditions, the usual form of young hatched from it, but other batches from the same female under abnormal conditions resulted in the production of atypical forms. For example, a normal two-brooded form became five-brooded. The commonest modification was the production of various color types. These once established, according to Professor Tower, behave as independent, inheritable units.
The experiments of Doctor Bardeen with X-rays and of others with X-rays, radium and other agents on the sperm and ova of amphibia show that these are very susceptible to injurious influence at or near the time of fertilization.
Such Effects Improbable in Warm-Blooded Animals.—However possible it may be to bring about germinal changes in invertebrata or lower vertebrata by such external agents as temperature and the like it is obvious that the probability of such extrinsic influences affecting the germ-cells of warm-blooded animals is very remote indeed. In the latter the germ-cells are more or less distant from the exterior and are at practically a constant temperature. Such experiments, therefore, beyond showing the possibility of producing changes in germ-cells, do not have very direct bearing on the problem of how inheritable variations are produced in man. In his case about the only avenue of approach through which germ-cells might be influenced is the blood or lymph.
Poisons in the Blood May Affect the Germ-Cells.—Any poisonous material in the latter might injuriously affect the gametes. We know, in fact, that such poisons as alcohol, lead and various drugs, and also the toxins of various diseases, do so affect germ-cells. It seems plausible to suppose that changing conditions of nutrition may affect the constitution of the germ-cells and thus induce changes in the organism which arise from these cells, but such nutritional effect is not yet a matter of established fact.
Difficulty of Explaining How Somatic Modifications Could be Registered in Germ-Cells.—As to our second query concerning the possibility of affecting the germ-cells through the intermediation of parental tissues, it is evident at a glance that since the germ-cells are built up along with the body and are not a product of it (Fig. 2, [p. 13]), if such effects are possible they must take place through the agency of some transporting medium. The germ-cells, being lineal descendants of the original fertile germ or zygote, already have the same possibilities of developing into an adult that the zygote had, and so the problem becomes one of modifying a complete germ already organized rather than of establishing a new germ by getting together samples of every part of the body. This is all the more evident when one realizes that usually the germ-cells are set apart long before the body becomes adult, that is, before the body has developed most of its characteristics. Moreover, among lower animals many instances are known where the immature young or even larvæ will produce offspring which nevertheless ultimately manifest all the structures of the adult condition.
But supposing specific modifications of the germinal mechanism were possible, it is difficult to comprehend how an influence at a distant point of the body could reach the germ-cell, to say nothing of the even greater difficulty of understanding how it could become registered in the germ in a specific way as affecting a particular part. For it must be remembered that the organs of the adult do not exist as such in the germ but are present there only as potentialities. How, for example, can a change in the biceps muscle of one’s arm be registered in a germ-cell in which there is no biceps muscle, but merely the possibilities of developing one? Or how can increased mental ability which is contingent on the elaboration of certain brain-cells be impressed on a germ which has no brain-cells but only the capacity under certain conditions of producing such cells? For the brain of a child is not descended from the brain of his parent, but from a germ-cell carried by that parent.
Persistence of Mendelian Factors Argues Against Such a Mode of Inheritance.—On the face of things, the apparent inviolability of Mendelian factors which may remain unexpressed in the germ for one or many generations—indeed the whole matter of genotypical differences in the gametes of the same individual—shows the improbability of somatic interference with the germ-plasm. But notwithstanding this, because of the great importance of the issue, it is well to review in some considerable detail the various phases and possibilities of the question.
Experiments on Insects.—Some of the attempts to secure evidence of the transmission of personally acquired parental modifications in insects are very interesting. Many insects in the larval stages, particularly just after pupation seem to be especially susceptible to external influences. They have been much used, therefore, for purposes of experiment. It has long been known that differences in size, in color and even in the shape of wings can be produced by various agents if applied at this period of development. From the standpoint of heredity, however, the important consideration is to determine if these experimentally induced changes have been reflected on to the germ-cells so that they reappear in the offspring of the modified individuals.
It has been found that in some cases where male and female are of different color, the color of the female can be changed to that of the male by altering the conditions of temperature. In certain cases types can be changed by cold so that they resemble varieties of the same species found farther north, and by heat, varieties found farther south. But not all individuals of a given lot are affected, and often different individuals of the same kind show different effects. Moreover, in some cases the same aberrations were produced by heat as by cold. This indicates that it is not so much a question of specific effects as a general physiological change, apparently mainly a matter of direct influence of temperature on the chemical composition of the pigments. The Countess von Linden in fact has shown that the extracted pigments can be made to undergo the same changes of color in a test-tube by heat and cold as in the pupæ. But there is no evidence that the germ-cells of the living insect were affected in a specific way. In a small fraction of the offspring of such modified individuals abnormalities appeared, but these were not always of the same kind as those which had been produced in the parent. That is, there was no evidence of a trait or character having been acquired by the body and handed on to the germ-cell. Where an effect was produced on the germ-cell it was probably produced directly as in the first cases discussed.
Size, colors and markings of butterflies have also been altered by subjecting the caterpillars or the pupæ to such influences as strong light, electricity, various chemical substances, centrifuging, diminished oxygen supply, etc., but the results were in the main confined to the immediate generations. In the few cases where permanent inheritable changes were seemingly produced they were more reasonably interpreted as the effects of direct action on the germ-cells than as examples of inherited somatic modifications.