Page:Catholic Encyclopedia, volume 5.djvu/741

 EVOLUTION

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EVOLUTION

The hybrids of any two diiTerent characters produce seeds, one half of which again develop the hybrid forms, while the other half yield offspring which re- rnains constant, and possess the dominant and reces- sive characters in equal proportion. A smiple analy- sis of this rule shows that it consists of three parts: (a) By fertilization the characters of the parents are united, without, however, thereby losing their purity and independence; (b) In the offspring the characters of both parents may again be separated from each other; (c) The character of one of the parents may completely conceal that of the other. This last part of the rule is not, according to later investigators, necessarily connected with the other two parts. We may add that Mendel's rule also holds good for the off- spring of hybrids, in which several constant characters are combined, and that in it there is found a splendid confirmation of the modern theorj' of the cell. Cross- breeding, therefore, does not by any means lead to the mixing of characteristics. These, on the contrarj', remain pure, or, at most, form new combinations or split up into simpler components. Hence, the idea that gaps in nature originate through such segregation is well founded. But the question, whether the idea is to be applied to the formation of species, and how this is to be carried out, can scarcely be answered at pres- ent. This much, however, is evident : that there is no progress in organization any more than there is anj' progressive specific development, brought about by segregation.

Hence this important conclusion follows: That the central idea of modern evolution theories — namelj^,

^ A ^

Al #ftii iiii ii

Qeii

progressive specific development — ha.s not up to the present received anj' confirmation from observation of the world of organisms as it now exists. It is quite true, however, that the plasticity of organisms has been proved by a number of experiments to be very considerable; so that, in a constant environment, and by single variations, changes may be brought about which a systematist would classify as specific or even generic, if it were not clear from other .sources that the}' are not such. In the same way fonns could be developed by segregation, the characteristics of which would suffice "to constitute specific differences in the eyes of most systematists, were the plants or animals brought home by collectors" (Bateson). Yet such criteria are meaningless for the demonstration of the fonnation of species. The question a.s to the trans- mission of acquired characters is not by any means decided. It follows from the doctrine of propagation that only such characters can be transmitted as are contained in the germ-cells or which have been either directly or indirectly transmitted to them. Hence it is clear that all peculiarities acquired by the cells of the body through the influence of environment, or by use or disuse, can only be inherited if they are handed

over, as it were, to the germ-cells. But it is useless to discuss the question before we have sufficient experi- mental evidence that acquired characters are at all inherited.

IV. The P.\l.eoxtologic.\^l Argiiiext. — (1) Uis- torical Method. Before entering upon the discussion of the evidence furnished by pala;ontologj' we must briefly refer to the method which ought to be em- ployed in the interpretation of the palceontological records. The great archives of the geological strata are verj' incomplete. Almost three-quarters of the earth's surface is covered with water, and another part with perpetual ice, while of the rest but a fraction has remained free from the ravages of water and the elements; of this small portion, again, only certain regions are accessible to the investigator, and these have been but partially examined. Besides, in most cases only the hard portions of organisms are pre- served, and even these are often so badly mutilated that their correct classification is sometunes difficult. Many of them, especially in the oldest rocks, must have perished under the crushing force of metamor- phic processes. Further, the geographic distribution of plants and animals must have varied according to climatological and topographical mutations. It may suffice to cite the glacial periods of which there are clear indications in various geological epochs. Finally, the geological strata themselves underwent many vio- lent strains and displacements, being upheaved, tilted, folded again, and even entirely inverted. It is evi- dent that everj' one of these phenomena increases the chaos in its own way and makes the work of classifying and restoring all the harder. It gives at the same tune to the scientist the right to formulate hj^jotheses probable in themselves and adapted to bridge over the numerous gaps in the work of reconstruction in the organic worUl. But these working hi,T30theses ought never to assimie the form of scientific dogmas. For after all, the dociunents which have really been deci- phered are the only deciiiing factor. At all events, the chronological succession and the genetic relation of organisms cannot be determined by aprioristic rea- soning, or by means of our present system of classifica- tion, or bj' apph'ing the results of ontogenetic studies. One illustration may suffice. Some maintain that trilobites are descended from blind ancestors because certain blind forms exhibit a number of simple charac- teristics which are common to all specimens. And yet we know that, e.g., Irinucletis possesses eyes in the earlier stages of its development, and only becomes blind in the later stages. The non-existence of eyes is, therefore, due to degeneration, and does not point to a former eyeless state. As a matter of fact, specimens of trilobites possessing eyes are fovmd side by side with eyeless specimens in the lower Cambrian strata. Other examples of false ^ priori conclusions are to be found in the extraordinary genealogies constructed by extreme evolutionists, and which dissolve like so many mists in the light of advancing uivestigations. In fact, up to the pre.sent the agreement on ontogeny and phylogeny has not been proved in any single in- stance. In .short, if we disregard observation and ex- periment on living organisms, it is the historical method alone which can decide the limits of evolution and the succession and genetic relations of the differ- ent forms. " In the substitution of the hypothetical ancestors by real ones lies the future of true phyloge- netic science" (Handli.sch).

(2) The Oldest Fosails. Now let us turn to the docu- ments ihem.selves and see what they have to show us. The foundation of the Archives is formed of gneiss and crystallized slate, a rigid mass containing no trace of organic life, and one which offers to the paheontologist the hopeless outlook that his .science must remain in a very incomplete state, perhaps forever. Immediately above this foundation, nature has imbedded the mul- titudinous, highly-developed Cambrian faima, with-