CHAPTER VII

THE INHERITANCE OF ACQUIRED CHARACTERS AS A FACTOR IN EVOLUTION

LAMARCK'S THEORY

One of the most striking and peculiar characteristics of living things is that through use a part is able to carry out a particular function better than before, and in some cases the use of the part leads to its increase in size. Conversely, disuse leads to the decrease of a part in size. We are perfectly familiar with this process in ourselves as applied to our nervous system and muscles.

It is not surprising that the idea should have arisen that, if the results of the use of a part are inherited by the next generation, the adaptation of organisms might be explained in this way. The presence of the organs of touch, in those parts of the body that are more likely to come into contact with foreign bodies, offers a striking parallel to the perfecting of the sensation of touch that can be brought about through the use of any part. The development of eyes only on the exposed parts of the body, as on the tentacles of the sedentary annelids, or along the margin of the mantle of a bivalve mollusk, suggests that there may be some direct connection between their presence in these regions and the effect of light on the parts. In fact, ever since the time of Lamarck, there have been many zoologists who have claimed that many of the adaptations of organisms have arisen in this way, that is, through the inheritance of the characters acquired through use. In general this theory is summed up in the phrase, "the inheritance of acquired characters."

This view is prominently associated with the name of Lamarck, who held, however, a different view in regard to the origin of some of the other structures of the organism. Moreover, Erasmus Darwin, even before Lamarck, had suggested the principle of the inheritance of acquired characters.

As has just been said, Lamarck held that the inheritance of acquired characters was only one of the ways in which animals have become changed, and he clearly stated that in the case of all plants and of some of the lower animals the change (evolution) which he supposed them to undergo was due to the general influence of the environment. Since plants and the lower animals (as he supposed) have no central nervous system, or at least no such well-defined nervous system as have the higher animals, Lamarck thought that they could not have evolved in the same way as have the higher animals. We now know that, so far as the lower animals, at least, are concerned, there was no need for such a distinction, since. many of their responses are like those of the higher animals. This distinction that Lamarck made is responsible, no doubt, for a misconception that was long held in regard to a part of his views. It is often stated that he supposed the desire of the animal for a particular part has led to the development of that part; while in reality he only maintained the desire to use a particular organ to fulfil some want led to its better development through exercise, and the result was inherited. Lamarck also supposed that the decrease in use of a part which leads to its decrease in size accounts for the degeneration of organs.

Lamarck first advanced his theory in 1801, when he cited the following examples in its favor. A bird, driven through want to the water to find its food, will separate its toes when they strike the water. The skin uniting the bases of the toes will be stretched in consequence, and in this way the broad membrane between the toes of ducks and geese has been acquired. The toes of a bird that is in the habit of perching

* Bots per progrem nature began, and begins sl Absproduce the simplest organams, and that she 11⁄2 une primitive kinds of organiza

a procna pas been called spontaneous generation.

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1 That the fire by aning of animals and of plants takes path in botas e Vocalities and under favorable circume An organic movement having once established fitur prodachon, they have of necessity gradually developed theat organa, and have become diversified in the course of time

4 That the power of growth of each part of the body being inherited as a consequence of the first effect of life,

different modes of multiplication and of regeneration have arisen, and these have been conserved.

5. That with the aid of sufficient time and of favorable circumstances the changes that have taken place on the surface of the globe have called forth new structures and new habits, and in consequence have modified the organs of the body, and made animals and plants such as we see them at the present day.

6. Finally, as a result of these changes that living bodies have been forced to undergo, species have been formed, but these species have only a relative constancy, and are not as ancient as is nature herself. If the environment remains the same, species also remain the same, as is exemplified by the animals living at present in Egypt, which are exactly like those living there in ancient times.

Lamarck concludes that the appearance of stability is always mistaken by the layman for the reality, because, in general, every one judges things relatively to himself. In fact, species are not absolutely constant, but are so only temporarily. "The influence of the environment is continuous and always active, but its effects may only be recognized after a long time." The irregularity and the complexity of the organization of animals is the outcome of the infinitely diversified circumstances to which they have been subjected. These changes, Lamarck claims, do not directly cause modifications in the form of animals,1 but bring about changes in their needs, and changes in their needs bring about changes in their actions. If the needs remain the same, the acquired actions become habits. These habitual actions lead to the use of certain parts in preference to others, and this in turn to an alteration in form and structure. The individuals so changed breed together and leave descendants that inherit the acquired modification.

Curiously enough, Lamarck follows up this argument by

1 This is clearly meant to be applied only in the case of higher animals.

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citing some cases amongst plants that have been changed directly by the action of the environment. He says that since plants have no motions they have consequently no habits, but they are developed by changes in their nutrition, etc., and this brings about the superiority of some of the vital movements over others.

Amongst domestic animals Lamarck cites the case of the dog, that has come from a wild form like the wolf, but having been carried into different countries has acquired different and new habits, and this has led to the formation of new races, such as the bulldog, greyhound, pug-dog, spaniel, etc.

Lamarck's argument shifts so often back and forth from animals to plants, that it is clear that in his own mind he did not see any important difference between the action of the environment on plants, and the use of the organs of the animal. He gives in this same connection his oft-quoted summary of what he calls the two laws of nature "which observation always establishes."

First Law. In every animal, that has not passed beyond the term of its development, the frequent and sustained use of any organ strengthens it, develops it, increases its size, and gives it strength proportionate to the length of time of its employment. On the other hand, the continued lack of use of the same organ sensibly weakens it; it deteriorates, and its faculties diminish progressively until at last it disappears.

Second Law. Nature preserves everything that she has caused the individual to acquire or to lose by the influence of the circumstances to which the race has been for a long time exposed, and consequently by the influence of the predominant use of certain organs (or in consequence of its continued disuse). She does this by the generation of new individuals which are produced with the newly acquired organs. This occurs, provided that the acquired changes were common to the two sexes, or to the individuals that produced the new forms.