Obrázky stránek
PDF
ePub

from a study of meteorological records. There is a climatic cycle of II years which corresponds with the increase and decrease of sun spots. A cycle of 35 years is also indicated by researches over a large part of the world, a period during which comes a series of cold or rainy years, followed by years which are warmer or drier. For instance, western Kansas, Nebraska, and eastern Colorado were settled near the close of a wet period (1880-1890), only to be abandoned as the dry cycle came on, and reoccupied as the supply of water increased.1

1 For a discussion of this subject and the bearing of climatic cycles on history, see Huntington's Pulse of Asia, Climate of Ancient Palestine, etc., and Bowman's "Man and Climatic Change in South America," Geog, Jour. (London), March, 1909.

CHAPTER VI

THE ORGANIC ENVIRONMENT

133. Adaptation to environment. Beside that section of the physical environment which might be called inorganic, and which has now been described in its several most significant phases, there exists yet another section, the organic. The influence of the flora and fauna of a region upon its human inhabitants is, as will be seen (§§ 149 ff.), unquestionably very great; and with a view to understanding the type of life lived by man in various parts of the world, it is necessary to treat somewhat of the natural distribution of plant and animal forms. The explanation of this distribution will also constitute the general case under which can be ranged the more complicated instance afforded by man's distribution (§§ 172 ff.).

What we desire at present to know about plant and animal life over the earth is, briefly speaking, how such variety came to exist. From a multitude of observations it is found that plants and animals are somehow adapted to the natural environment in which they live: desert flora will store moisture and do not easily lose it, polar animals have thicker and longer fur than their congeners of warmer zones, it is often white in color, or even changes color with the seasons, and so on. To catalogue the correspondences between organic life and the inorganic environment would be an endless task; certain typical cases, besides, will occur when we come to consider the influence of flora and fauna on man (Chapter VIII). It is more profitable here to outline with brevity the general causes which lie behind the production and distribution of the several types of flora and fauna.

134. The struggle for existence. First of all, organic life is subject to increase at a geometrical ratio- to an increase so rapid that the slowest reproducing of organisms would speedily fill up the earth with their offspring were there no hindrances to their multiplication. An annual plant producing only 50 seeds a year would become the ancestor, in the ninth year, of some 1,950,000,000,000,000 plants; if each of these occupied one square foot of soil, the whole surface of the globe would then be occupied, and some 500,000,000,000,000 seeds would be left

over with no place for their growth. Animals producing ten pairs annually to each pair (each animal living ten years) would show over 700,000,000,000,000,000,000 pairs alive in the twentieth year. Yet these rates of possible increase are not large: consider the number of acorns on the oak, or of eggs in the roe of fish. But with all this possibility of increase we know that such accession does not take place; in general, the numbers of the plants and animals, taking them, say, over a hundred years, fluctuate around an average; they are now greater, now less, but in the long run stationary. Hence we must infer a tremendous death rate of all organisms; but since the first business of any organism is to live, death will not be accepted without a struggle. Plants will choke each other out; and animals, with a more generally recognized "instinct for self-preservation," will not succumb while there is a chance to resist. This is the struggle for existence; and it is so severe, as can well be conceived, that even a slight natural advantage or disadvantage may decide the issue of life and death. Weakness of any kind may prove fatal; hence any group of organisms, which even though they weather the struggle for a time, yet do so at a sacrifice of strength, have no surety as the competition renews or grows fiercer (cf. §§ 138-140).

135. Survival of the fitter. But no two organisms really have the same chance in this struggle, for no two are alike. There are two factors in life which operate, the one to render offspring different from parents and from each other, and the other to keep the type the same. The former is variation, the latter heredity. Heredity produces its effects by transmitting ancestral variations more or less perfectly from generation to generation; thus, while offspring vary, sometimes slightly, sometimes greatly (mutations), from parents, in general they all belong to a single type with certain more or less unique characters. Armed now with these principles, let us return to our struggle for existence. Certain types will prevail because of certain characters which enable them to fit into the environment and endure its conditions; the rest will die or degenerate. The survivors will produce the next generation, which will tend to possess by heredity the winning qualities in the struggle - which never ceases. Those of the second generation possessing the most marked variations in the happy direction will again survive and breed, and so on. This is, of course, an age-long process. Thus will the possession of the fortunate qualities be fixed and their degree of advantageousness be increased. Those organisms which possess these qualities will be better adapted to live; in their persistence will be seen the survival of the fitter.

Of all the bear cubs born in a cold region some will tend to develop coats warmer than those of others; their chance to live is greater; they will tend to produce young which show the same variation toward better protection. Under severe competition in the struggle for existence we should, perhaps after some centuries, have a development of the thick, long fur characteristic of the polar bear. This type would be the fitter.

136. Modification and adaptation. If, now, it is the fitter organisms which survive, then, should the environment be changed, in order to remain fit the successful organisms must undergo further modifications of structure. In short, the whole process which has been sketched leads up to structural modifications on the part of plants and animals, to changes calculated to set them, or to keep them, in harmony with their natural environments. The whole of the above is briefly resumed in the following (Wallace's) chart of the evolutionary process 1:

[blocks in formation]

Now it is precisely these structural modifications upon which the various genera, species, and varieties of plants and animals are distinguished; that is, these adaptations to environment are what make the type or character of the organism. Here, then, we have the how of the matter of plant and animal differences, whereby to keep in mind our especial interest — the lower organisms are able to affect in one peculiar way or another the life of man; whereby they constitute a condition of human environment, varying in different localities.

137. Dispersal. A single alternative in the struggle for existence stands over against death, degeneracy, or adaptation; and that is flight. The migration of birds with the seasons is only a swifter flight than the slow movement of forests along with temperature belts, during geologic ages. Rather than die or adapt, the arctic plant has ascended the upper ridges of tropical mountains, and the ancient animals, now of a polar, now of a tropical type, have deserted central Europe. But

1 The teacher will appreciate the fact that so brief a sketch of so large a subject as is attempted in the present chapter must be greatly condensed; and that much which is implicit in the above chart, and in the several paragraphs of the chapter, should be made explicit in the class-room discussion.

That the evolutionary process does not result in superlative but only in comparative fitness is indicated by the bracketed emendation of the chart.

flight has not always been the destined alternative; under the impulsion of some necessity, or because there was, after all, less resistance along the way of adaptation than otherwise, plant and animal life has, through the ages, penetrated all possible regions of the globe, and, becoming adapted to this and that environment, climatic and other, has come to constitute the characteristic types of various regions. The how of this diversification, so wide reaching in its influence on man, is now explained, at least in its most general outlines. Something of the influence upon man of this diversified organic environment, itself so dependent upon the "inorganic," as well as something of man's own form of adaptation, rendered more easily explicable now that the process of plant and animal adaptation has been indicated, will come before us in the succeeding chapters.

[ocr errors]
« PředchozíPokračovat »