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mass of shells, there a piece of petrified wood, an insect in the marl bed or a leaf preserved flat in the shale. Each of these fossils is a record of past life, true beyond impeachment, but the fragments are so few, so scattered, so broken, as to give only hints of the history they represent.
Moreover, as we extend our studies of species we find that they change with space as well as with time. These changes
are in large degree a response to external conditions. As conditions change, so do forms change to fit their surroundings. A movement over the surface of the earth, any movement in space, brings organisms in contact with barriers. A barrier means a change in conditions of life. As distance in space brings barriers, so does the passage of time bring events which are barriers also. Time brings new events; events mean changes in conditions, and change brings about divergence. Neither time nor space flows evenly.
Variations in turn become greater with lapse of time and space, for these again bring other events and disclose other barriers. A closer observation will show us that the range of variety is far greater than is indicated by the number of species. There is not one blade of grass in the meadow exactly like any other blade. There is not a squirrel in the forest like any other squirrel, not a duck on the pond like any other duck in every detail of its structure. If we compare two rose leaves we shall find differences in size, in serration of the margin, in the length of the stalk, in the hairs on the surface, in the intensity of the green, in the number of breathing pores on the lower side. In every structure and function where difference is possible variation will appear. The squirrels or the ducks will differ in shade of color, in distinctness of marking, in length of limb, in breadth of organ, in every way in which there is play for individualism.
Nor are these differences limited to matters of color or form. There are like variations in function, in tendency, in disposition, in endurance. No two men ever bore the same features, no two ever held the same character, no two ever lived the same life. The traits of the in
Fig. 8.- Sea cucumber, Cucudividual, however small, appear on maria, sp. (Natural size.) every hand. It is by little traits of emphasis that we recognize our friends. The same individualism is possessed by the lower animals and by plants, though the differences in stress and emphasis in color and figure are most marked in creatures of the most highly specialized organization. In all animals and all plants like differences obtain. No two individuals of any species are ever quite the same. No two germ cells of the same parent are ever quite alike. No two cells in the body are ever exactly identical. Among plants of the same kind in the field, some are cut down by frost while others persist; some are destroyed by drought while others endure; some are immune to attacks of rust while others are ex
Fig. 10.—White pelicans, Pelecanus erythrorhynchus, and whooping-crane, Grus ameri
cana. (Photograph by W. K. Fisher.)
terminated by such parasites. Fill a bottle with flies. All in time will die of suffocation, but a certain few will outlast the many. Bring in a number of wolf cubs. Some will become relatively tame—some will remain wolves, and between the most fierce and the most docile we shall find all ranges of variation. “What is one man's food is another man's poison." This proverb is a recognition of the principle of individuality which accompanies everywhere the formation of species, and being everywhere present, it must be an integral part of the
process. Such differences are not matters of structure alone. Psychological differences, differences in instinct, in adaptability, in rate of nerve processes are just as marked as differences in anatomy. They may separate one species from another. They may be just as decided within the limits of the species itself. Moreover, the beginning of variation is not with the individual organisms. No two cells are absolutely alike, and in the variance of the germ cells, from which individuals spring, all the elements of their future variation are involved.
Without further discussion, it is evident that variety in life is a factor in the history of our globe, that it may be expressed in terms of number of species, but that the actual range of variation is far greater than the number of species, and that if causes are to be judged by range of effects, we must find in the origin of species the operation of world-wide forces, the coöperation of great influences, far-reaching in time and space, as broad as the surface of the globe and as enduring as its life. To consider these causes so far as known is the purpose of this work. Our problem is no longer the “mystery of mysteries," for in a large way by the work of Darwin and his successors the influences promoting variety in life are already known. We know many of the different factors which produce divergence in form and adaptation to conditions. But the relative value of these factors is less certain, and from time to time other and more
Fig. 12.-Lizard walking. (After Marey.)
subtle factors are brought to light, or the great forces themselves are analyzed into finer component elements.
But with all that we may say of the universality of variation and the prevalence of individualism, we are equally impressed with the underlying unity. There are only a few types of structure among animals, and in these few the beginnings in development are the same. The plants show similarly a few modes of development, and all the range of families and forms is based on the modification of a few simple types. Moreover all living forms, plants and animals alike, agree in the fundamental elements. All are made of a framework of cells, each cell a source of energy, containing in all cases a semifluid network of protoplasm, which is found wherever the phenomena of life appear. In all the cells is the mysterious nuclear substance which seems to direct the operations of heredity. The same laws or methods of heredity, variability, and response to