The idea that germline cells contain information that passes to each generation unaffected by experience and independent of the somatic (body) cells, came to be referred to as the Weismann barrier, and is frequently quoted as putting a final end to the theory of Lamarck and the inheritance of acquired characteristics. While Weisman based the idea on his limited knowledge of cells and his (largely wrong) theory of Germ Plasm, he is also widely quoted as having ‘proved’ the non-existence of Lamarckian inheritance by the experiment of chopping of the tails of fifteen hundred mice, repeatedly over 20 generations, and reporting that no mouse was ever born in consequence without a tail. In fact he states that ‘901 young were produced by five generations of artificially mutilated parents and yet there was not a single example of a rudimentary tail or any other abnormality of the organ’.[4] Despite the fact that this experiment has been repeatedly declared invalid, both then and today, many modern textbooks continue to declare that Weismann ‘proved’ that inheritance of acquired characteristics does not occur. It is also often reported that his experiments were not needed since generations of Jews and others had practised circumcision, without any observed effect on offspring. In fact Weismann was aware of the limitations of his experiment, and made it clear that he embarked on the experiment precisely because, at the time, there were many claims of animals inheriting mutilations (he specifically refers to a claim regarding a cat that had lost its tail having numerous tail-less offspring). There were also claims of Jews born without foreskins. None of these claims, he said, were backed up by reliable evidence that the parent had in fact been mutilated, leaving the perfectly plausible possibility that the modified offspring were the result of a mutated gene. He carried out his experiment in order to lay such claims, regarding mutilation specifically, to rest. What Lamarck and others actually claimed was the inheritance of characteristics acquired through necessity, or effort, or will, or environment, which is quite different.
In fact, Weismann, who knew almost nothing of the complexities of modern genetics, was in no position to declare such a barrier, especially since proving that something cannot happen requires investigation of every conceivable mechanism by which it might just happen. The philosopher Henri Bergson explained this well as long ago as 1911 in response to Weismann’s assertion when he said, in his book Creative Evolution: After having been affirmed as a dogma, the transmissibility of acquired characteristics has been no less dogmatically denied, for reasons drawn a priori from the supposed nature of germinal cells. It is well known how Weismann was led, by his hypothesis of the continuity of the germ-plasm, to regard the germinal cells - ova and spermatozoa - as almost independent of the somatic cells. Starting from this, it has been claimed, and is still claimed by many, that the hereditary transmission of an acquired character is inconceivable. But if, perchance, experiment should show that acquired characteristics are transmissible, it would prove thereby that the germ-plasm is not so independent of the somatic envelope as has been contended, and the transmissibility of acquired characters would become ipso facto conceivable; which amounts to saying that conceivability and inconceivability have nothing to do with the case, and that experience alone must settle the matter.[5] Bergson goes on to describe the difficulties that arise in such experiment, of separating the effect of habit from a natural aptitude that may have existed to induce the habit.
The dogmatic nature of Weismann’s assertion was seriously challenged in the 1980s and 1990’s by the Australian immunologist, Ted Steele[6]who presents considerable evidence for what is now known as soma to germline transfer in his book Lamarck’s Signature. There has been a recent revival in what is known as Neo-Lamarckism