In organic evolution, when the struggle for life is fierce and pitiless as it is now among the higher animals, natural selection is undoubtedly by far the most potent factor. It is at least conceivable (though not probable) that at the present time organic evolution might be carried on mainly or even wholly by this factor alone; but in human evolution, especially in civilized communities, this is impossible. If Weismann and Wallace be right, then alas for all our hopes of race improvement—physical, mental, and moral!—for natural selection will never be applied by man to himself as it is by Nature to organisms. His spiritual nature forbids. Reason may freely use the Lamarckian factors of environment and of use and disuse, but is debarred the unscrupulous use of natural selection as its only method. As this is an important point, we must explain.
All enlightened schemes of physical culture and hygiene, although directed primarily to secure the strength, the health, and the happiness of the present generation, yet are sustained and ennobled by the conviction that the improvement of the individuals of each generation enters by inheritance into the gradual physical improvement of the race. All our schemes of education, intellectual and moral, though certainly intended mainly for the improvement of the individual, are glorified by the hope that the race also is thereby gradually elevated. It is true that these hopes are usually extravagant; it is true that the whole improvement of one generation is not carried over by inheritance into the next; it is true, therefore, that we can not by education raise a lower race up to the plane of a higher in a few generations or even in a few centuries: but there must be at least a small residuum, be it ever so small, carried forward from each generation to the next, which, accumulating from age to age, determines the slow evolution of the race. Such are the hopes on which all noble efforts for race-improvement are founded. Are all these hopes baseless? They are so if Weismann and Wallace are right. If it be true that reason must direct the course of human progress, and if it be true also that selection of the fittest in the organic sense is the only method which can be used by reason, then the dreadful law of pitiless destruction of the weak, the helpless, the sick, the old, must with Spartan firmness be voluntarily and deliberately carried out. Against such a course we instinctively revolt with horror, because contrary to the law of our spiritual nature.
But the use by reason of the Lamarckian factors is not attended with any such revolting consequences. All our hopes of race-improvement, therefore, are strictly conditioned on the efficacy of these factors—i. e., on the fact that useful changes, determined by education in each generation, are to some extent inherited and accumulated in the race.
CHAPTER IV.
SPECIAL PROOFS, TAKEN FROM THE GENERAL LAWS OF ANIMAL STRUCTURE, OR FROM COMPARISON IN THE TAXONOMIC SERIES. General Principles.
Analogy and Homology.—In biology those organs or parts in different animals are said to be analogous which, however different their origin, have a general similarity of form and especially of function; while those are called homologous which, however different their general appearance, and however different their function, yet may, by close examination and extensive comparison, be shown to be modifications of one another—to be, in fact, originally the same part modified for different purposes. In the former the parts compared look and behave as if they were the same, but are not; in the latter they look and behave entirely differently, but are, in fact, the same part in disguise.
We can best make this plain by examples. The wing of a bird and the wing of a butterfly are analogous organs. They have the same function—i. e., flying; and this function necessitates the same general form of a flat plane. But they are not at all homologous; they are not at all the same organ or part. They certainly have never been formed one out of the other by modification. But the wing of a bird, the fore-paw of a reptile or mammal, the wing of a bat, and the arm and hand of a man, though so different in form and function, are homologous parts. On close examination they are found to have the same general structure, to be composed of essentially the same pieces, although they are so greatly modified in order to adapt them to different functions, that the general or superficial resemblance is now lost. Their structure is precisely such as it would be if they had all originated from some archetypal fore-limb by modifications in different directions of its several parts. By extensive comparison in the taxonomic and ontogenic series, all the intermediate gradations between these extreme modifications may be picked up.
Fig. 2.—Lepidosiren.