Part 166. Acquired Adaptations - Kinship And Adaptation

Part 166. Acquired adaptations. The belief that all existing organisms are the more or less modified descendants of relatively primitive forms, is now as generally held by naturalists as is the belief in universal gravitation among astronomers. Yet astronomers are still striving to understand how it is that the force of gravity can act as it does; likewise naturalists are still debating the fundamental question as to how inherited modifications have arisen. That somehow plants and animals have evolved is now taken for granted; but there are wide differences of opinion as to the way in which the changes have been wrought.

In these few pages we can glance only briefly at the leading views now held regarding the origin of species. Each theory aims to account for the appearance of those peculiarities of structure or behavior which distinguish one group from another; as for instance the climbing habit of clematis and the long, mostly hairy-tailed fruit which is found nowhere else in the family except with certain anemonies inhabiting wind-swept fields. Peculiarities of this sort are often connected in some helpful way with the life of the individual-hairy appendages insuring to achenia widespread dissemination by wind, and the power to climb facilitating economically the quick exposure of green parts to sunlight. In such ways an organism, as we have seen, is adjusted, often marvelously well, to its environment; and we call the adjustments which promote its welfare adaptations. It is with questions of the origin and inheritance of adaptations that the debates of evolutionists are mainly concerned. We must, therefore, at the outset examine carefully just what is meant by an adaptation.

Every individual plant or animal is found to have the power of modifying its form or behavior in response to outside influences, and such modification is often beneficial. Indeed we need not distinguish here between form and behavior, for even structure is but the result of growing in certain ways, and growth is merely a slow kind of behavior. So we may say that each living thing, in so far as it is alive, has a certain limited power of adaptation through direct response to environing influences. As a seed sprouts, its little root turns toward the place of greatest moisture, while the young leaves are directed toward the light, and if the illumination be feeble the stem helps the leaves by elongating more than it would if the light were stronger. A tree exposed to strong winds of one prevailing direction takes on a onesided form thereby reducing the strain. Herbs which in rich moist soil produce tall stems and ample foliage before they flower, will on a sandy roadside bloom as soon as they have made a few small crowded leaves and are only a few inches high, thus, doing the best they can under adverse conditions. Dandelions grown on a mountain side look very different from those grown in the lowlands (Fig. 301), and the peculiarities of each seem to fit in especially well with the contrasting conditions. Such cases of advantageous adjustment made during the life of an organism may be termed individual adaptations. From these we must distinguish characteristic adaptations, or advantageous peculiarities belonging to whole groups. Of such adaptations the characteristics of clematis above referred to may serve as examples; or we might take a mountain species of buttercup (Fig. 302) which differs from near relatives of the lowlands in having a stunted, compact form well suited to alpine conditions and a rosette of leaves somewhat resembling those of our mountain form of dandelion. A peculiarity supposed to have arisen in response to some direct influence of the environment is said to be acquired. If it be characteristic of a group it is called an acquired character, while in so far as it serves to fit the organism for the special conditions of its life it is an acquired adaptation.

Fig. 301.-Common Dandelion (Taraxicum officinale, Sunflower Family, Compositce). P, plant as it grows in the lowlands, reduced in size. M, plant as it grows at high altitudes, reduced in the same proportion as P. M', the same plant as M less reduced. (Bonnier.)-Native home, Europe; very common as a weed in America.

Fig. 302.-Pigmy Buttercup (Ranunculus pygmoeus, Crowfoot Family, Ranunculaceoe). Plant. Flower. Fruit. (Britton and Brown.)- Perennial (?) herb 5-12 cm. tall; flowers yellow; fruit dry. Native home; northern America and Eurasia.

It has been thought by many naturalists that the differences among plants and animals may best be accounted for as peculiarities which have arisen in individual acquirements, these having become hereditary and thus characteristic after many generations of similar response to a similar environment. Suppose, by way of example, that some of the seedlings from primitive marsh-marigolds grew in a rather dry locality and were more or less shaded by overgrowth. They might be expected to respond to the lessened light by elongation of the internodes and leaf-stalks, while a finger-like lobing of the blades through increased growth along the ribs and scant growth of the pulp between the stem would be fortunate as enabling the leaves to catch more of what little light there was. But the less favorable conditions for food-making would render it impossible for them to form and feed as many seeds as the marsh plants had done; hence some of the later-formed ovules would be more or less starved. From the good seeds formed by these pioneer plants and scattered in the vicinity, a second generation would arise, the individuals of which would respond similarly to the same trying conditions. Innumerable generations might follow, similarly responding, and if each generation were influenced ever so little by the responses of the generations before, that is to say, if the acquired peculiarities were inherited to any extent, the foundations laid by the pioneers would be built upon by their descendants. As a result we should have in successive generations the stems growing longer, and the leaves more branched and finally becoming compound. An increased sensitiveness to light might be accompanied by greater sensitiveness to contact and this might lead to a coiling of the leaf-stalks around neighboring twigs, thus establishing the habit of climbing by means of which better exposure to light would be most economically secured through utilizing for support the very bushes which had made the shade. As a consequence of the starving out of the upper ovules there would result finally a fruit the upper part of which had become a mere tail-like projection surmounting a one-sided indehiscent base. A general hairiness which had been developed in all the exposed parts of the plant in response to the dryness of its new environment might especially affect the fruit as being now especially exposed, and might lead even to an elongation of the tail which would thereby become well adapted for enabling the wind to carry the precious seed high over surrounding shrubbery. In some such way as this it is conceivable that the characters of a clematis may have evolved.

The first naturalist to suggest that organisms had evolved through the accumulation of acquired characters was Jean Baptiste Lamarck, of France, who flourished in the early part of the nineteenth century. His doctrine is called Lamarckism, or as modified by his more recent followers, Neo-Lamarckism. Lamarckians have advanced much evidence to show that acquired characters are often adaptive and may be inherited; but while most naturalists might concede the possibility of such characters being now and then adaptive, the great majority of evolutionists have remained unconvinced that acquired characters are ever fixed by inheritance. So far as we know, acquired characters do not long survive the conditions under which they arise. Cultivated plants escaped from cultivation soon become, as we have seen, indistinguishable from wild plants of the same species. But might not very long subjection to changed conditions finally fix an acquired character? Possibly, and it should be said that the botanical evidence is perhaps more favorable to this view than the zoological; but if the facts can be explained without relying upon suppositions hardly possible to verify, we may build upon a safer foundation.