Validity of the August Weismann mouse-detailing experiment refuting Lamarckism

Question

Weismann conducted the experiment — described in Wikipedia — of removing the tails of 68 white mice, repeatedly over 5 generations, and reporting that no mice were born in consequence without a tail or even with a shorter tail. This was intended to refute the Lamarckian idea of of acquired characteristics.

However, surely five generations is too short a time to observe a change in an organ when the fossil record indicates that such changes in animals took millions of years. Was this not known at the time and does it invalidate the experiments?

Answer 1

If you are reading this after 31st July 2017 it will seem somewhat disconnected from the question. This is because the question has been extensively edited since I wrote this answer.

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Anyone who is interested in the work of August Weismann should start by reading the corresponding Wikipedia page.

Ernst Mayr ranked him as the second most notable evolutionary theorist of the 19th century, after Charles Darwin.

As far as the mouse tail experiment is concerned, we learn that Weismann was concerned with disproving Lamarckianism, and that:

Weismann was aware of the limitations of this 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 refers to a claim regarding a cat that had lost its tail having numerous tail-less offspring).

My grandfather used to claim that his mother was frightened by a bear whilst pregnant, which was was why he was born with bare feet but that's more likely to be due to an epigenetic effect I suppose.

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Added later in response to Remi.b

I see that Charles has already addressed this to some extent, but since I have already done the research I'll post what I found:

from August Weismann, Essays Upon Heredity. Clarendon Press, Oxford, 1889

(Volume 1, Chapter 8, p. 431)

But now, after this little digression, let us return to the transmission of mutilations.

We have seen that the rudimentary tails of cats and dogs, as far as they can be submitted to scientific investigation, do not depend upon the transmission of artificial mutilation, but upon the spontaneous appearance of degeneration in the vertebral column of the tail. The opinion may, however, be still held that the customary artificial mutilation of the tail, in many races of dogs and cats, has at least produced a number of rudimentary tails, although, perhaps, not all of them. It might be maintained that the fact of the spontaneous appearance of rudimentary tails does not disprove the supposition that the character may also depend upon the transmission of artificial mutilation.

Obviously, such a question can only be decided by experiment: not, of course, experiments upon dogs and cats, as Bonnet rightly remarks, but experiments upon animals the tails of which are not already in a process of reduction. Bonnet proposes that the question should be investigated in white rats or mice, in which the length of the tail is very uniform, and the occurrence of rudimentary tails is unknown.

Before this suggestion was made, I had already attacked the problem experimentally. Such a course might, perhaps, have been more natural to those who maintain the transmission of mutilations, to which I am opposed. Although I undertook the experiments expecting to obtain purely negative results, I thought that the latter would not be entirely valueless; and since the numerous supporters of the transmission of acquired characters do not seem to be willing to test their opinion experimentally, I have undertaken the not very large amount of trouble which is necessary in order to conduct such an experimental test.

Answer 2

As has been mentioned, the study that you're referring to was conducted as to debunk the notion that mutilations (acquired characters) of a parent were able to be inherited by their offspring (referred to as Lamarckian theory). More specifically, Weismann set out to demonstrate that only germ cells contribute to inheritance. That, somatic cells are produced by germ cells, but that somatic cells do not influence the production of germ cells. Today we believe this to be true, and refer to it as the "Weismann barrier".

To provide a few quotes of Weismann from his Essays Upon Heredity:

If acquired characters cannot be transmitted, the Lamarckian theory completely collapses, and we must entirely abandon the principle by which alone Lamarck sought to explain the transformation of species,--a principle of which the application has been greatly restricted by Darwin in the discovery of natural selection, but which was still to a large extent retained by him.

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I believe that I am able to show that the actual existence of transmission of acquired charaters cannot be directly proven; that there are no direct proofs supporting the Lamarckian Principle.

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It can hardly be doubted that mutilations are acquired characters: they do not arise from any tendency contained within the germ, but are merely the reaction of the body under external influences. They are, as I have recently expressed it, purely somatogenic characters, viz. characters which emanate from the body (soma) only, as opposed to the germ-cells; they are therefore characters which do not arise from the germ itself.

So, the (currently accepted to be) mechanics of evolution and heredity most definitely weren't known during that time, but instead were just being discovered/established! As can be observed from the first quote, even Darwin wasn't fully convinced that Lamarckian theory was incorrect, though it directly conflicted with his own findings. Also during that time, Mendel was alive and conducting his own experiments. Pretty cool, huh??

Lastly, here's another (this time loosely translated) quote from Weismann regarding Lamarckian theory; one that I like quite a bit:

If you came across a case of an inherited acquired character, it's as if a man sent a telegram (written in English) to China and it arrived translated into Chinese.

Answer 3

[D]oesn't it take around millions of years for an organ to disappear or appear

No, not necessarily. But whether this claim is true or not is not so much the point of your question.

False Negative Result

Weismann did not observe an effect of cutting a rat tail on the length of the tail of rats at birth. It is what we call in statistics a negative result. Any negative result can be a true negative (cutting rat tail really does not affect the length of the tails in the offspring) or a false negative (maybe Weissmann had too a small sample size or maybe the experience should have been conducted on a longer time scale).

This is true for any experiment. It is the basis of the philosophy of knowledge. You can't show evidence of absence, you can only show evidence of existence. So, if an effect does not exist (if the result should be negative), then the only thing one can do is to fail to show evidence for the existence of this thing.

So, Weismann was in no way naive. Just like any other scientist, he just accepted the very nature of how knowledge can be acquired about the world. Actually, from @AlanBoyd's answer, Weissmann did comment on the limitations of his study.

This section was more about philosophy of knowledge than about biology. You should maybe learn a bit more about philosophy of knowledge, philosophy of science and typically about some key concepts such as Russel's teapot and evidence of absence.

So was it a false or a true positive

It was a true positive! We today understand a lot about genetic mechanism of inheritance and know that modifying the phenotype will not affect the DNA sequence (although it may affect the surrounding of the DNA sequence but this is a story for another time).

Introduction to genetics

You might want to follow an intro course such as Khan Academy > Classical and molecular genetics, Khan Academy > DNA as the genetic material and Khan Academy > Central dogma.