Evolution and Adaptation

by Thomas Hunt Morgan

In the cases amongst animals, where the maternal tissues are believed to be influenced by a previous union with the male, as in the oft-cited case of Lord Morton’s mare, a reëxamination of the evidence by Ewart has shown that the case is not demonstrated, and not even probable. Several years ago I tried to test this view in the case of mice. A white mouse was first bred to a dark male house-mouse, and the next time to a white mouse, but none of the offspring from the second union showed any trace of black. If the spermatozoa of the dark mouse are hypodermically injected into the body-cavity of the female, the subsequent young from a white male show no evidence that the male cells have had any influence on the ovary.

The following facts, spoken of by Darwin himself, are not in favor of his hypothesis of pangenesis: “But it appears at first sight a fatal objection to our hypothesis that a part of an organ may be removed during several successive generations, and if the operation be not followed by disease, the lost part reappears in the offspring. Dogs and horses formerly had their tails docked during many generations without any inherited effect; although, as we have seen, there is some reason to believe that the tailless conditions of certain sheep-dogs is due to such inheritance.” The answer that Darwin gives is that the gemmules themselves, that were once derived from the part, are still present in other parts of the body, and it is from these that the organs in the next generation may be derived. But Darwin fails to point out that, if this were the case, it must also be true for those cases in which an organ is no longer used. Its decrease in size in successive generations cannot be due to its disuse, for the rest of the body would supply the necessary gemmules to keep it at its full state of development. Thus, in trying to meet an obvious objection to his hypothesis, Darwin brings forward a new view that is fatal to another part of his hypothesis.

The following cases, also given by Darwin, are admitted by him to be inexplicable on his hypothesis: “With respect to variations due to reversion, there is a similar difference between plants propagated from buds and seeds. Many varieties can be propagated securely by buds, but generally or invariably revert to their parent forms by seed. So, also, hybridized plants can be multiplied to any extent by buds, but are continually liable to reversion by seed,—that is, to the loss of their hybrid or intermediate character. I can offer no satisfactory explanation of these facts. Plants with variegated leaves, phloxes with striped flowers, barberries with seedless fruit, can all be securely propagated by buds taken from the stem or branches; but buds from the roots of these plants almost invariably lose their character and revert to their former condition. This latter fact is also inexplicable, unless buds developed from the roots are as distinct from those on the stem, as is one bud on the stem from another, and we know that these latter behave like independent organisms.” As Darwin here states, these facts appear to be directly contradictory to his hypothesis, and he makes no effort to account for them.

The entire question of the possibility of the inheritance of acquired characters is itself at present far from being on a satisfactory basis, as we shall try to show; and Darwin’s attempt at an explanation, in his chapter on pangenesis, does not put the matter in a much more satisfactory condition.

The Neo-Lamarckian School

Let us now turn our attention to a school that has grown up in modern times, the members of which call themselves Neo-Lamarckians. Let us see if they have supplied the essential evidence that is required to establish the Lamarckian view, namely, that characters acquired by the individual are transmitted to the offspring.

Lamarck’s views were adopted by Herbert Spencer, and play an important rôle in his “Principles of Biology” (1866-1871), and even a more conspicuous part in his later writings. In the former he cites, amongst other cases, that of “a puppy taken from its mother at six weeks old who, although never taught ‘to beg’ (an accomplishment his mother had been taught), spontaneously took to begging for everything he wanted when about seven or eight months old.” If tricks like this are inheritable is it not surprising that more puppies do not stand on their hind-legs?

The larger hands of the laboring classes in England are supposed to be inherited by their children, and the smaller hands of the leisure classes are supposed to be the result of the disuse of the hands by their ancestors; but even if these statements in regard to size are true, there are many other conceivable causes that may have led to this result.

Short-sightedness appears more often, it is said, in those classes of society that make most use of their eyes in reading and in writing; but if we ask for experimental evidence to show that this is due to inheritance, and not due to the children spoiling their eyes at school, there is none forthcoming. The problem is by no means so simple as the uninitiated may be led to believe.

Spencer thinks that “some of the best illustrations of functional heredity are furnished by mental characteristics.” He cites the musical faculty as one that could not have been acquired by natural selection, and must have arisen through the inheritance of acquired modifications. The explanation offered is “that the habitual association of certain cadences of speech with certain emotions has clearly established in the race an organized and inherited connection between such cadences and such emotions, ... and that by the continued hearing and practice of melody there has been gained and transmitted an increasing musical sensibility.” But a statement that the results have been acquired in this way does not supply the proof which the theory is in need of; neither does it follow that, because the results cannot be explained by the theory of natural selection, therefore, they must be explained by the Lamarckian theory.