Popular Science Monthly/Volume 33/June 1888/Animals as Modified by Environment
|ANIMALS AS MODIFIED BY ENVIRONMENT.|
By J. B. STEERE,
PROFESSOR OF ZOÖLOGY IN MICHIGAN UNIVERSITY.
THE close connection between animals and their surroundings is generally recognized both by those believing in creation by design and by those holding to evolution. This connection is usually supposed to be restricted to the adaptation of certain organs to specific facts of surrounding environment. Often-quoted examples of such related organs and conditions are the eye and light, and the ear and sound.
In addition to this undoubted adjustment of single organs to individual conditions of environment, there is reason for believing that each natural division of the great animal types, the most fundamental as well as the most trivial, is adapted in the same sense to its own special fact of environment. In other words, all modifications of type have been in the line of adaptation to special conditions; and, where such modifications are
Abstract of a paper on "The Importance of Individual Facts of Environment in the Formation of Natural Groups of Animals," read before the American Association for the Advancement of Science, Ann Arbor meeting, 1885. distinct enough to be recognized by the classifier, the facts of environment on which they depend should be distinct enough to be discovered by the observer of animal habits.
This proposition can be established only by the connection of structure with habits and with conditions of environment in a large number of groups of different values in each of this types. Its reasonableness is best shown by the fact that it is recognized as true in those groups whose habits and structure are best known. The difficulty of establishing it as a general truth lies rather in the lack of knowledge of animal habits and surroundings than in a lack of knowledge of structure.
A few undoubted examples of adaptation of groups to special environment are of birds to aërial habitat, of fishes to water, of rodents to hard foods, and of squirrels to arboreal seeds with hard coverings.
One of the difficulties in tracing this connection between existing groups and the environment to which they are adapted is in this, that the more fundamental structural characteristics may remain after the animal possessing them has, by later superficial modifications, become adapted to other and perhaps antagonistic conditions, and even after the conditions leading to such structures have disappeared. Of the first case we have such examples. as the ostrich and penguin, which, while retaining their bird characters, have lost flight, and have become, one of them terrestrial and the other aquatic; and the bats and whales, mammals which are no longer capable of existing in the normal mammalian habitat.
Of examples of the last case, animals existing after the conditions leading to their existence have disappeared, we can not be so sure; but the marsupials and the proboscidians among mammals, and the turtles among reptiles, may be examples in point. If animals may become superficially modified so that they may exist under conditions different from those for which they were primarily fitted, they might still exist after such primary conditions had ceased to exist. Whales might exist if all land were destroyed.
Another difficulty in connecting animal forms with special conditions of environment is in the multitude and variety of modifications that have taken place. No type of animal life has stopped at one set of changes. If there was one species of bird, and that fitted in a general way for all bird-life, one of mammals, one of fishes, etc., the problem would be simple; but as soon as a group of animals has become adapted to a new fact of environment it falls under the influence of a new set of surroundings, more or less numerous, from the very fact of such change, and again becomes broken up into subdivisions adapted to each of these. This process is repeated again and again until groups of animals are reached in which the differences of structures separating them from others become so minute that they are called varietal or specific while the differences of environment to which these subdivisions are adapted grow fainter and fainter, until, when the point is reached where the classifier is compelled to throw together a number of animals and call them a species, because he can no longer find structural differences to form divisions upon, there the student of animal habits will find the same animals related to practically identical facts of environment.
An example of such changes in environment and of such following modifications of structures may be taken from the mammals. Primarily arboreal in habits, and climbing, with hands developed on one or both pairs of limbs, their skulls and jaws and teeth were fitted for masticating their food. All of their supposed ancestry had simply swallowed their food whole.
This mammalian character of the teeth, when attained to, made of importance, for the first time, such differences of foods as necessitated different kinds of teeth for masticating them. Examined in this respect, the food ordinarily made use of by mammals may be roughly divided into five classes—fruits, insects, flesh, grass, and hard foods—and these have led to the formation of the five great orders: primates, insectivores, carnivores, ungulates, and rodents. The importance of these food characteristics in modifying mammals may be seen when it is stated that these five orders, with the bats, which are flying insectivores, contain five sixths of the mammalian species and probably more than nineteen twentieths of all the individuals of the class. Again, the becoming fitted for hard foods, by the rodents, brought them under the influence of a new set of surroundings, namely, the various locations in which such foods existed, and the families of squirrels, mice, hares, beavers, etc., are the result. The same process again took place in the formation of the genera and species of these families.
We must look upon the class, order, family, genus, and species characters of each individual animal as structures which fit it or have fitted its ancestors for as many distinct facts of environment. The later gained and more trivial structures may overlie and obscure the more ancient and fundamental ones, like the later writings on an old palimpsest, but all are to be made out by the skillful anatomist. As has been said, the line of modification is not always a direct one, but it is often so deflected that structures which were primarily adapted to one fact of environment may be modified secondarily to fit them for others which are opposed and antagonistic to the first. The examples of swimming and running birds without flight, and of marine and aërial mammals, have already been given. In these cases, the fundamental structures become in part worn and mutilated, and some of them entirely lost. Some of the running birds have lost a large part of the bones of the wing, and the whales have lost the hinder limbs. Such mutilation and loss of parts is proof that the animal which has suffered them has departed from the environment of its ancestors.
But though deflection and antagonism of structures is possible and frequent, it is necessarily not usual; later modifications of structures are ordinarily in harmony with more fundamental ones, and later conditions of environment with primary ones. Progress is ordinarily easiest in a straight line. Most fishes live in the water and swim, though they become variously modified for the various secondary conditions found in this medium. Most birds fly, though they are subjected to endless modifications which are in harmony with flight. The mass of shot show the spot aimed at, and not the scattering pellets. When later modifications are in agreement with primary ones, the primary structures remain in full perfection and use.
The facts of environment bearing upon life are so various and so heterogeneous that they allow of but little classification. Those conditions which have existed pretty generally over the earth, and with little or no change since the creation of life, have had the most profound modifying influence. Among these are the different mediums respired, air and water, and the different horizons or locations requiring peculiar organs of locomotion, deep and shallow water, earth, rocks, and trees, and air. The divisions of types which are usually dignified by systematists with the title of classes have their reason for existence in conditions of this kind. The five recognized classes of vertebrates—fishes, amphibians, reptiles, birds, and mammals—are simply modifications of type for life in water, in shallows, on land, on trees and rocks, and in the air.
But these conditions are not confined to this first influence upon the types. Being in continual and unchanged existence, they again had their influence, among other conditions, in forming the secondary divisions of the type, that is, the groups called orders, and have caused many or all of the deflections of these from the class-lines of structure. In the mammals the order of whales and bats, and in a less degree the ungulates, are cases in point. Among the birds, the ostriches, with the ordinary wading, running, and swimming birds, are examples. Even in the orders the influence of these great primary conditions is not lost, but with less and less power, as the specializations of family, genus, and species are reached, they still show their force in such families as the beaver, in such genera as the otter and muskrat and flying-squirrel, and in such species as the water hare of the Southern States.
In addition to the class of conditions above mentioned which have led to fundamental differences of breathing organs and of organs of locomotion especially, there are others which have had a much more superficial effect in modifying life. These are such as foods, enemies, climate, and perhaps others too subtile to be known at present. These differ from the first in having come into existence, many of them, since the creation of life, and in having been in a continual state of change. The groups called orders, families, genera, and species, depend on these in most cases for their reason for existence. The structures modified lead to differences of size, color, shape, teeth, or other organs of food taking, and secondary modifications of the organs of locomotion. Like the first class of conditions mentioned, these may also by their continual existence cause modifications in other groups than those especially and primarily fitted to them. The primates are fruit-eaters and the bats are normally insect-eaters; but the fruit-bats are secondarily modified in teeth, size, stomach, etc., for fruit-eating.
The groups founded on these secondary and changing conditions have also been in a corresponding state of change, old forms disappearing and new ones taking their place. Where facts of this second class have approached stability, the groups corresponding have partaken of this character in the same degree. The foods which became such important factors in the modification of mammals must have at a very early period taken on the general characters of fruits, flesh, insects, grass, and hard substances, and the great orders were at an early period formed and have remained, and must do so while the earth exists in anything like its present state.
Many of the so-cailed families are also based upon conditions which have a good degree of permanence; but as the lesser groups are approached the facts of environment upon which they are established become more and more narrow and more capable of either destruction or change. While the general class of hard foods may remain as long as terrestrial life exists, particular kinds of nut-trees or of grains may disappear, and with them species and even genera depending upon them for existence. As the conditions leading to the formation of the lesser groups grow more narrow in their character, being limited perhaps by a single species of food, the location in which this condition exists becomes restricted also, and so the chances for its destruction are increased. But very many of the changes among the ultimate groups are not by destruction, but by change of the conditioning environment slow enough to he accompanied by change of the species depending upon it. There can he but little doubt that many if not most or all of existing species are slowly changing in mass and in place into something else than what they are now. This process may be so slow that it will require some such careful study and delicate calculation as has been used by astronomers to prove that the so-called fixed stars are in motion; but the well-founded belief that geological changes are still going on upon the earth is enough to make us certain that changes ' in living things must accompany these or follow close after.
In addition to such slow changes of the mass of individuals of a species as must finally produce a form specifically different from it, there is another method of formation of species which would seem to be much more prolific in its results, and which alone can keep the earth populated during ages of changes of environment, some of them so rapid and violent that they destroy great numbers of species. This is by the migration of individuals into surrounding areas where the conditions are so near like those to which they are accustomed that they can exist, and still different enough to set up rapid changes of structure. The offshoots of the parent species might thus become numerous, and still retain likeness enough to each other so that they would be thrown by the systematist into a common genus; or, if the changes of environment were less, we should have a set of subspecies or varieties. There is no line or rule fixed in nature by which we can say that this is a genus with several species, and this is a species with several varieties. If this is a true theory for the creation of species, there should exist certain species settled and fixed in character, which have existed with but the slow modifications of structures caused by geological changes, while other species would be ready to change in any direction, or to revert rapidly to the characters of its parent species. The varieties and species of man's make are exaggerated examples of the latter class, and man himself would furnish illustrations of the same thing in such ancient and well-established species as the Chinese, and such a recently-formed variety as the Anglo-American. The heterogeneous character of the conditions of environment bearing upon life, and their utter lack of equality or equivalence of modifying power, give good and sufficient rear son for that lack of equality of structural values among groups, which is best shown by the inability of the best systematists to agree upon their value. The fate of the group called species is a case in point. As long as the animals of a single well-defined area were studied, there was no difficulty, for closely-allied species necessarily rarely or never inhabit the same area; but as soon as the study became comparative, by bringing in forms from neigh boring areas, species lost its significance as a term of fixed meaning, and can not be separated from genus on one side and subspecies or variety on the other.
Though the examples made use of in this discussion are taken from the vertebrate type of animals, the other great divisions fall under the same law, each in its own way, and under limitations set by the characters of the types themselves.