Starvation experiments which resulted in the dwarfing of adult individuals have been performed on various insects, and while the dwarf condition may persist through one or two generations due to a diminished food supply in the eggs of the dwarf, the stock in question when returned to normal food conditions soon resumes its original characteristic size.
Experiments on Plants.—Many experiments have been performed with plants, inasmuch as they are particularly prone to become modified by changes of food supply, or climate. For example, plants which grow luxuriantly in a warm moist climate or a rich soil may become stunted and markedly changed if transplanted to a cold climate or a poor soil. Naturally, their progeny will exhibit the same behavior as long as they are kept under the new conditions. Experiments carried on through numerous generations, however, practically all show that the germinal constitution of the plants remains unchanged, for when their seeds are planted under the original favorable conditions of soil or climate, the plants resume their former habits of growth. Naegeli, for instance, who made a study of many varieties of Alpine plants, and who carried on experiments with many of them for years in the Garden of Munich, concluded that no permanent effects had been produced by the Alpine climate and conditions in plants from other regions which had come under its influence. A few botanists have claimed to have found that the changes produced by the Alpine climate have persisted for a generation or two and have then worn off. More recent experiments on various of our field grains which have been stunted and cut down in productivity by growing for a number of generations under adverse conditions show that they have not been permanently modified by such treatment, for they resume normal productivity and size when grown again under favorable conditions.
On the other hand, Lederbaur found that a common weed, Capsella, when transplanted from an Alpine habitat to the lowlands did not return to the lowland type of the weed, but retained certain of its Alpine characteristics. It is not clear, however, that this particular species during its long sojourn of many generations in Alpine conditions may not have undergone a series of germinal variations and have developed into a new variety or species quite independently of changes wrought in the germ by reflected somatic effects. Indeed, in face of the preponderance of other cases to the contrary, this interpretation would seem to be the more plausible one.
Experiments on Vertebrates.—In the vertebrates we may also find examples of various somatic modifications experimentally produced, but evidence of their inheritance is as difficult to establish as in the invertebrates. Let us examine a few of the more significant of these which are alleged by some to bear evidence of such inheritance.
By decreasing the amount of water in an aquarium Marie von Chauvin was able to transform the aquatic, gill-breathing salamander Axolotl into the gill-less land form Ambystoma, heretofore regarded by systematists as a different species. Either of these forms when sexually mature produces its like. The salamanders in question have both lungs and gills, but after a time the ones which are to be land forms lose their gills and become exclusively lung-breathers. What seems to have been accomplished then is the accelerating or forcing of normal natural tendencies already inherent in the organism instead of introducing something new into the inheritance by way of the soma. Axolotl is in all probability merely a larval form of Ambystoma which with high temperature and an abundance of water reproduces without advancing to the final possible stage of its life cycle.
Epilepsy in Guinea-Pigs.—Perhaps the most frequently cited case and the one in which the defenders of the idea of somatic inheritance usually take final refuge is that of Doctor Brown-Sequard’s guinea-pigs, notwithstanding the fact that no one has had convincing success in repeating the experiments and that the original results are apparently open to more than one interpretation. This experimenter rendered guinea-pigs epileptic by certain injuries to the nervous system. Epilepsy appeared in some of the offspring of these operated animals. He regarded this as an example of the inheritance of an artificially induced epilepsy. An indirect loss of toes occurred in some of the parents as a result of the operations on the nervous system. Some of their young also had missing toes. However, as has been pointed out by various critics, guinea-pigs are strongly predisposed toward epileptic-like seizures, and the epilepsy in the young may have been merely a coincidence. Voison and Peron believe they have shown that in epilepsy a toxin is produced that may affect the unborn fetus. That is, the result might have been due to a poison derived directly from the mother. The experiments in fact show that it was mainly in the offspring of affected mothers that the condition appeared. Others maintain that we do not know the exact nature of epilepsy, that in some cases it may be the result of infection by disease-germs, and that Brown-Sequard’s cases may, therefore, have been merely the communication of a disease from parent to child. As to the disappearance of toes it is a well-known fact that rodents in particular are likely to gnaw off the toes of their young very soon after birth, and little credence can be put in a lack of toes in such young as cases of inheritance except under conditions of much more careful observation than existed in Brown-Sequard’s experiments. A fuller account of these experiments will be found in Romanes’ Darwin and After Darwin, Vol. II, Chap. 6.
Effects of Mutilations Not Inherited.—Many experiments have been performed by investigators to determine whether or not the results of mutilation are transferred to succeeding generations, but so far only with negative results. Many such experiments have been unwittingly carried on for many generations, in fact, by breeders and fanciers, in the docking of horses, dogs and sheep, the dehorning of cattle and the like, yet no satisfactory evidence of the transmission of such conditions in any degree has ever been forthcoming. The mutilations or distortions of the human body through various rites or social customs also fails to yield any convincing examples. Foot-binding, head-binding, or waist-binding must be repeated in each successive generation to produce the particular type of “beauty” that results from such deformities. And lucky it is for man that injuries do not persist in subsequent generations, otherwise the modern human being would be but a maimed relic of past misfortunes.
Transplantation of Gonads.—An interesting experimental test regarding the effect of the body on the germ was made recently by Castle and Phillips with guinea-pigs. It will be recalled from the discussion on Mendelism that when a black guinea-pig is mated with a white one the offspring are always black. These experimenters transplanted the ovaries from a young black guinea-pig to a young white female whose own ovaries had been previously removed. This white female was later mated to a white male. Although she produced three different litters of young, six individuals in all, the latter were all black. That is, not a trace of coat-color of the white father or of the white foster-mother was impressed on the transplanted germ-cells or the developing young. Later experiments of the same kind by Castle and Phillips, with other varieties of guinea-pigs, have yielded the same results. The body of the mother, indeed, seems to serve merely as a protective envelope and a source of nutrition.
Effects of Body on Germ-Cells General, Not Specific.—As far as the evidence regarding the modification of the germ-plasm by the body is concerned, we must conclude then that while under special circumstances the germ-cells may be affected, the effect is general rather than specific and the result as seen in the offspring has no discoverable correlation with any particular part or structure of the parental soma. The effect is presumably of much the same nature as where the germ is directly affected by external agents. Where a new character or a modification of one already existing is produced by a given condition of environment, in our experience so far to have the same repeated in the offspring, a similar evocative condition must prevail in the environment of the latter. Or in other words the new character is not a permanent one which persists in succeeding generations independently of external influences similar to those which originally produced it.
Certain Characters Inexplicable as Inherited Somatic Acquirements.—It would require remarkable credulity, in fact, to believe that some of the most striking features about certain plants or animals could have been developed by means of the inheritance of somatic modifications. For example, many animals such as the quail, the rabbit, or the leaf-butterfly are protectively colored. That is, they harmonize in color-pattern with their surroundings so closely that they are overlooked by their enemies. But how can this oversight on the part of an enemy so affect the bodies and through them the germ-cells of such individuals as to develop so high a degree of protective coloration? Or how, indeed, could any of numerous adaptive structures which one can think of, such as the color or scent of flowers to lure insects for cross-pollenation, the various grappling devices on many seeds to secure wide distribution by animals, or the like, have been directly produced by use or disuse or by any variation produced in them by the agents to which they are adapted?