Lamarckism

In his

Origin of Species, Darwin accepted the principle of the inheritance of acquired characters as one of the factors contribu tory to evolution. In his later book on Animals and Plants under Domestication, Darwin elaborated his view and proposed a "pro visional hypothesis" to explain the transmission. This hypothesis he called Pangenesis. Each part of the body is imagined to throw off invisible particles called gemmules, which, getting into the blood-stream, are supposed to collect in the germ-cells, and there combine with like units already present, modifying them in ac cordance with the changes that have taken place in the peripheral organs. Hence the next generation arising from the germ-cells is a photograph, as it were, of the parent at the particular stage when the germ-cells were formed. This hypothesis, frankly tenta tive, recalls an ancient belief recorded by Hippocrates that all parts of the body contribute to form the semen of the male, the egg furnishing nourishment for its development. It is, perhaps, almost needless to point out that this vague conception of the mode of development of the germ-cells is totally at variance with modern knowledge concerning the origin of eggs and spermatozoa, which in many cases are present and often developed before the adult stages are reached. The most that might be claimed would be that the units of heredity already present might be modified by late contributions from other parts of the body.

A. Weismann's challenge of the entire theory of the transmis sion of acquired characters marks a turning-point from the older credulous attitude to the modern demand for more critical evi dence. Weismann's challenge was not based, however, on experi mental evidence. His few experiments—cutting off the tails of mice—can scarcely be regarded as convincing. Darwin had, in fact, pointed out that the loss of a part is not inherited, because he added, the germ-material already contained the inherited units for the development of each part. Nevertheless, the admission weakens the argument that organs become rudimentary through disuse. Weismann succeeded in showing the complete inade quacy of much of the evidence generally accepted by Lamarckians in support of their views, and he pointed out, moreover, that the continuity of the germ-plasm—an idea suggested earlier by Gal ton in his theory of the stirp—suffices to account for the inheri tance of the great majority of characters common to all individuals of a species without the need of postulating contributions from the body-cells.

In more recent times the discussion has been carried on by several palaeontologists, Hyatt, Cope, and at one time Osborn. In fact, as early as 1853 Lyell had reported Lamarck's views and reprinted them in later editions of his famous Principles, treating the evidence, however, with great caution and scepticism. The

doctrine has also appealed to some psychologists and popular writers, owing to a fancied resemblance of heredity to memory (Hering, Samuel Butler q.v.). Others, impressed by the difficulty of explaining co-ordinated reactions and instincts unless nervous reactions are inherited, have resorted to Lamarck's doctrine. The most recent advocate of something of this sort, McDougall, has tried to show that trained rats show at least some degree of transmission of a newly acquired character, but his results have been criticized on valid statistical grounds. The inheritance of ac quired characters was vigorously maintained by Herbert Spencer. In more recent times special applications of the doctrine have been made by Semon in his book on the Mneme, and by Pauli in his volume Darwinismus and Lamarckismus. Delage, in his L'Heredite, gives a critical summary of the evidence, advocating a limited acceptance of the Lamarckian principle, as does Romanes in his second volume on Darwinism. Thomson in his popular treatise on Heredity has not supported the hypothesis. Cuenot in his brochure L' Adaptation has pronounced against it, while Cun ningham has emphasized the need of some such theory of heredity to account for sexual dimorphism, and later has appealed to hor mones as furnishing a possible means of communication between the body and the germ-cells.

Experimental evidence for and against Lamarckism has come, in still more recent times, conspicuously to the front on several occasions. This evidence covers a great diversity of subjects and a variety of methods, opening up new possibilities, some of which are far removed from the original contention. One line of evi dence goes back to the extraordinary results of Browne-Sequard with guinea-pigs. Injury of one of the parents, localized or even more general, caused, he believed, epilepsy in the offspring, or deformation and sloughing off of a limb or toes, and these effects were, he thought, transmitted sometimes to a few of their descend ants. The results have, in general, not been confirmed by subse quent work, although a few physiologists have given partial sup port to his conclusion though with important reservations. Rom anes has summarized much of the evidence in the second volume of his book on Darwin and After Darwin. It is obvious at the present time that, unless work of this sort is done with inbred and pedi greed material, any conclusion is venturesome in the extreme; for, unless it can be demonstrated first, that some of these peculi arities are not present in the stock from the beginning, the results may be due to germinal inheritance of recessive characters, or to indirect injury to the germ-cells themselves. The lack of speci ficity in the outcome may seem to support the latter interpreta tion rather than legitimate Lamarckism.