Читать книгу The Theories of Darwin and Their Relation to Philosophy, Religion, and Morality - Rudolf Schmid - Страница 12

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PRESENT STATE OF THE DARWINIAN THEORIES.

§ 1. The Theory of Descent.

The historical retrospect of the Darwinian theories, from their purely scientific side, leads us of itself to a critical review of their present state. We can briefly indicate in advance the result to which it will lead us, viz.: that the descent theory has gained, the selection theory has lost ground, the theory of development oscillates between both; but that all three theories have not yet passed beyond the rank of hypotheses, although they have very unequal hypothetical value. We can best arrange our review by beginning with that theory which is the most common, and which perhaps may still have value when both the others find their value diminished or lost: the theory of descent. From that we proceed to the theory of evolution, and from this to that of selection.

The theory of descent is indeed at first sight exceedingly plausible, and will probably always be the directive for all future investigations as to the origin of species. The organic species show, besides the great variety of their characteristics and the unchangeable nature of these characteristics, many other qualities which are common to them; and these common characteristics are precisely those which are most essential. Moreover the higher the structure of the organisms which are differentiated, the more numerous and more valuable will become the evidences of similarity, and the greater also will be their distance from the inorganic and from the lowest organisms of their class, their type, or their kingdom. For instance, rose and apple-tree, elder and ash, wolf and dog, goat and sheep, ape and man, are not only a great deal farther removed from the mode of existence of inorganic bodies than the algæ, the monera, and other low organisms, but they have also, in spite of the great interval which separates them from one another and especially which separates man from every animal, much more numerous and important points of contact than, for instance, two families or genera of algæ or of mosses, of polyps or of infusoria, have among one another. Now our imagination refuses to accept the theory that the Creator, or nature, or whatever we wish to call the principle generating the species, in producing the new species, laid aside all those points of contact which are continually becoming more numerous and more important, and produced instead, by ever widening leaps, the new and higher species from the inorganic, which lies farther and farther from them. On the other hand, the theory appears to us all the more plausible, that every new species came into existence on that stage which is the most nearly related to it, and which was already in existence. If we add further, that the two old maxims of the natural scientists, omne vivum ex ovo and omne ovum ex ovario, have not been invalidated, in spite of all the searching for a generatio æquivoca, and that, even if the origination of the lowest organisms out of the inorganic could in future be proved, yet the truth of these maxims for all the higher organized individuals is established as a fact without exception. Moreover, if we take into consideration the fact that we can not at all imagine either the origin or the first development of a higher animal or a human organism without the protecting integument and the nourishing help of a mother's womb, we may venture to say that each and every attempt to render the origin of the first individuals of the higher species conceivable, leads of necessity to the descent theory. We have either to reject, once for all, such an attempt, as an unscientific playing with impossibilities, or to accept the idea of descent. It is certainly the lasting merit of Darwin, even if his whole structure of proofs should in the course of time show itself weak, that he not only had the courage (as others had before him), but also inspired scientists with the courage to trace the idea of a descent of species in a scientific way.

To be sure, so long as we have no other proof of the descent theory than the circumstance that we can imagine it, it will continue to be nothing more than an ingenious hypothesis. We have, therefore, to look to the realm of nature for more direct proofs; and we are there furnished with them. They are presented to us by geology in connection with the botanical and zoölogical systems, by geology in connection with vegetable and animal geography, by comparative anatomy, and by the history of the embryonic development of animals.

Geology finds in the strata of the crust of the globe a large number of extinct plants and animals of extraordinary variety; but all of them, however much they may differ from the organisms of to-day, are completely in harmony with the botanical and zoölogical systems in which we divide the still living organisms. Not only have by far the most of the now extinct genera and species their family and stem-companions, and many even their genera and species companions, in the living world, but also those genera whose nearer relations are now extinct—as, for instance, the club-moss-trees, the trilobites, the ammonites, the belemnites, the sauria, the nummulites—show still a very perceptible relationship with living genera, and can be quite accurately included in the botanical and zoölogical systems; nay, they even fill up gaps in it. The anatomical, morphological, and, so far as we can judge, the physiological and biological relationship of the fossil with living organisms, is so great and comprehensive that in the present state of science a systematic botany or zoölogy, that should only treat of the fossils or of living organisms alone, would be imperfect. But the relationship of the fossil organisms with the natural systems of botany and zoölogy is apparent not only in this respect, but also in the fact that the single species during the long periods of time which are shown by geology to have elapsed, came into existence in a series, which again pretty closely corresponds to the natural system of the organic kingdoms; and that the fossil representatives of all classes and families, the nearer they come to the present world, appear the more nearly related to the living organisms, so that the fauna and flora of the ante-human time are lost in those of the human period by transitions gliding from the one to the other. For instance, in the Miocene formation of the tertiary epoch we find thirty per cent. of species still living to-day; in the Pliocene, even sixty to eighty per cent., and toward its end even about ninety-six per cent. of species which are identical with those now living.