LAMARCK'S THEORY The problem of the "inheritance of acquired characters," that is, of the possibility or the reverse of the transmission to his off spring of a variation from the normal mode of a species which an individual owes, not to anything innate in the fertilized egg from which he himself arose, but to the influence of the environ ment, either directly exercised or represented by a reaction on the part of the animal, has claimed the attention of leading zoologists since Lamarck postulated it as an essential part of his theory of evolutionary mechanism.
This hypothesis was based on the belief that every animal tends to change its structure and habits during its individual existence in such ways that it becomes better fitted for life under the conditions to which it is subjected. If the characters so ac quired be transmitted to its offspring, which in turn become adap tively modified, there will in time arise a group of individuals dif fering "specifically" from the original progenitor. Thus the ac quired characters whose inheritance was postulated were of the special character of "direct adaptations" to definite environmental conditions, and represent special responses of the animal. Un fortunately, the reliable evidence in favour of such inheritance is extremely scanty, and no case is of exactly the nature required by Lamarck.
A number of zoologists have conducted experiments to test the possibility of the hereditary transmission of the results of injury.
The vast numbers of experiments, made unintentionally by breeders, in the cropping of the ears and tails of dogs and other domestic animals, and the similar operations on man, have yielded negative results. Deliberate experiments of the same kind have been equally unsuccessful. But it may be argued that the reaction of the animals to such mutilation, involving merely the formation of a new skin over the wound, are not comparable to those which result in a direct adaptation.
Very few attempts have been made to alter the character of an animal by changing its environment, and to test the heritability of the modifications. Experiments on the desert mouse, Peromys cus, give no positive results, and a series of experiments claiming to establish such a transmission in the case of Amphibia and rep tiles is under very grave suspicion, and has not yet been repeated.
The most satisfactory experiments are of an entirely different type. During the latter part of the i9th century it was observed that, in certain districts, South Lancashire, West Yorkshire, Northumberland, North Kent, in England, in some of the indus trial districts of Germany, and near Pittsburgh, U.S.A., some moths were gradually changing their appearance. The change con sisted in the occurrence of melanic individuals, in which the wing pattern became obscured by a nearly uniform black coloration.
Such types were at first rare, but they gradually increased in num ber to such an extent that the original types of some of the species can no longer be found in some of these areas. It is quite certain that this occurrence of melanism is causally connected with the in dustrialization of the areas in which it is found, because, in coun try districts, the original wild type remains unaccompanied by the black forms. The change was formerly accounted for by a natural selection of the favoured melanics, but the origin of the variation was left unexplained.
Dr. Heslop Harrison has attempted to discover the cause of this melanism by a series of experiments based on the suggestion that it is induced by the direct action on the animal, or on its contained germ cells, of the mineral salts which are deposited from smoke on all the food plants in industrial areas. These experiments were carried out, in part, with a moth which had never been known in a melanic form, and consisted in feeding the caterpillars on clean food as a control, on plants collected in a smoky, "melanic" area, and on plants artificially infiltrated with manganese or lead salts.
During the course of the experiments melanic individuals appeared amongst the moths resulting from the larvae which had been fed on the contaminated food, and their descendants, although fed on clean food, still exhibited the melanism whose inheritance con formed to the Mendelian type.
It is to be noted that this melanism was fully developed at its first appearance, and that, although it represents a reaction of the animal to the chemical agent applied, it has not been shown, and it is most improbable that this reaction is of any value to the individuals which present it. Thus the primary postulate of Lamarck is not met. Furthermore, it is not certain that this melan ism is an "acquired character" in the ordinary meaning of that term. It is, perhaps, more probable that it results from a direct action of the metal ions on the germ cells, and that hence the original melanic individuals possessed that character from their initiation at fertilization, and would have displayed it even if they had been fed throughout on untreated food.
Indeed, it is perhaps legitimate to regard this case as a parallel to the effect of X-rays in increasing the rate of production of mutations in Drosophila. These experiments, though they have every appearance of reliability, require confirmation, because the number of cases in which melanism has been induced is very small. There is, therefore, no single case in which an inheritance of an "acquired character" of the type which is required by a Lamarckian explanation of evolution can be proved to have oc curred.