II. ADAPTATIONS IN ANIMALS AND PLANTS, WITH EXAMPLES

ADAPTATION

T is common knowledge that animals and plants respond

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to their environment, that is, to the physical influences exercised over them, and this gives them the power to

continue to exist. Many of these responses are purely physiological and temporary, but some of them result in permanent structural change. Romanes states, in regard to plants, "that it may fairly be doubted whether there is any one species of plant whose distribution exposes it to any considerable differences in its external conditions of life, which does not present more or less considerable differences as to its characters in different parts of its range." This is due largely to the effects of climate, the chemical and mechanical nature of the soil, temperature, intensity and duration of light, moisture, and the presence of certain salts in the air, as well as more unknown causes. Similarly, as a necessary part in the maintenance of existence, every animal adjusts itself to the surroundings. It must adapt itself to the food it finds, the air it breathes, as well as the climatic conditions. In consequence of these adjustments, modification with descent causes them to be transmitted from generation to generation. Of these vital adaptive characters, natural selection is presumed to be the important factor in their preservation. Those characters of a supposed non-essential type, such as some color markings on butterfly wings, due to variation, are supposed to be maintained by heredity and aided by isolation. By isolation, Romanes says, is meant "the prevention of intercrossing between a separated section of a species or kind and the rest of that species or kind, whether such a separation 1"Darwin and After Darwin," Part II, p. 206.

be due to geographical barriers, to migration, or to any other state of matters leading to exclusive breeding within the separated group."

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Numerous examples of special adaptations are given throughout this book. For instance, in the tree toad, which often selects the bark of trees to rest upon, the color changes so as to simulate exactly the colors of its surroundings, its body often appearing like a rounded excrescence on the bark. All the grasshoppers, notably the katydids, such as shown in the colored plate of the cone-head (facing page 22), are colored in simulation of their surroundings.

Then we have the walking-stick whose attenuate body resembles the twig of a tree. The humming-bird's bill and tongue are so modified as to gain ready access to flowers such as the columbine. Again, the modifications of the biting mouthparts of grasshoppers and the suctorial tongue of the butterflies offer examples of adaptations familiar to all. It is readily conceived that the front legs of the mole cricket and those of the ordinary garden mole have been adapted and perfected to a particular kind of environment, of tunnelling under the ground.

While the conditions under which organisms of a specialized type remain the same, the species may be able to maintain themselves, but it is readily seen that extermination would likely follow any radical changes in the environment.

PLANT ADAPTATIONS

In the dispersal of seeds of plants many contrivances have become specialized to assist in carrying them to places favorable to their germination and growth. The winged seeds of the maple and elm, the small, downy, and parachute-like structure of the milkweed and wild lettuce, present broad surfaces to the wind by which the seed is carried abroad. Others are provided with hooks, such as the burdock, and often become attached to the fur of animals. Again, there are many hard seeds that are contained in eatable fruits which are devoured by birds and mammals, and pass through

1 Post-Darwinian questions in "Darwin and After Darwin," Part III, p. 2.

PLANT AND INSECT ASSOCIATION OF THE MEADOW IN SEPTEMBER

The blazing star (Lacinaria spicata) with its insect visitors, the butterfly (Argynnis bellona), and the clear-wing moth

(Hemaris axillaris). From a photograph

their bodies unaffected. These seeds are favored in their germination by the heat and moisture to which they have been subjected, and perhaps later derive nourishment from the excreta passed with them. The fruits bearing the seeds often possess bright colors as they ripen and are charged with agreeably tasting juices which render them edible to animals. Some of these fruits are poisonous to some animals, but harmless to others. The bright orange mountain-ash berries are much enjoyed by the common waxwing; the black elderberries are consumed by several species of birds; and Wallace states that there is probably nowhere a brightly colored pulpy fruit which does not serve as a food for some species of bird or mammal. The drupes of the poison ivy are freely devoured by crows and in this way the plant is widely disseminated in our woods.

While the seeds are often dispersed as above described, it is an interesting fact, as Grant Allen has mentioned, that the fruits of our forest trees are protected during their development. At this time they are green when on the tree and hardly visible among the foliage, but as they ripen they turn brown and fall to the ground. Such, for instance, are the beech, butternuts, chestnuts, and the walnuts. The beechnuts and chestnuts are provided with a prickly coat that protects them to some extent, while the butternuts and walnuts have an acrid, pungent covering before they are ripe. I have seen young red squirrels eat nearly ripe beech and butternuts, biting through and discarding the outer covering. Out of the enormous quantities of these nuts that are produced some are doubtless able to propagate and produce young trees. The wild pigeons that formerly ranged over North America in such enormous flocks fed on acorns, which they swallowed whole without bruising, and these were digested and used up in nutriment. Squirrels and gophers are fond of acorns, and these nuts are destroyed by the acorn weevil, yet with all the great number of them consumed, there are enough produced that some of them find favorable places for germination and growth.

The smaller plants, such as grasses, sedges, composites, and umbelliferas, drop their seeds directly to the ground, and these have obscurely colored capsules and small brown seeds. Plants,