Page:The New International Encyclopædia 1st ed. v. 14.djvu/327

* NATURAL SELECTION. 283 NATURAL THEOLOGY. Thi.s appears to be pi-actically tlic siiiiic view as that whicfi was advaiiopd bj' Lamaick, and which is caUiMl by Kinici' '(iitliogcnosis' (q.v.). Variation as the result of obaiigod conditions is ill nearly every case — perhaps four-llflhs of all that occur, lluis Icaviiio; very little scope for the plaj" of chance or fortuitous variations — due to causes of which we are ignorant. Fortuitous variations, in fact, are an almost iic;;lif;ible quan- tity. Hence the primary postulate of natural .selection that variations are in general fortuitous is in the nature of an assumption, and not based on observed facts. As the result of recent in- vestigation we are coming more and more to the view that variation in general is the result of the action of clianged conditions of life, condi- tions both physical and biological, and that nat- ural .selection is not an active agent. (8) Since Darwin called attention to the lack of long series of intermediate links between sjie- cies, naturalists- have been more and more in- clined to the liclief that such series of connect- ing variations have never existed, but that na- ture makes leaps, that species often arise by sud- den or 'quick' or saltatorial evolution. Certainly neither in the Paleozoic or later strata, nor at the present time, do we observe these series of minute- ly graduated numberless anneetent forms postu- lated by Darwin, there still being gaps between the known connecting links. Hyatt. V. H. Dall, Galton. De Vrics. and many others, have advo- cated the view of the rapid or sudden modifica- tion of struiture involving the sudden appear- ance of species, especially in Paleozoic times. This is seen by the results of the examination of the Steinheini forms of Planorbis. and of the Tertiary forms of Austrian Paludina. where there are often wide gaps even between the connecting links. The new species of evening primrose raised by De 'ries originated suddenly, w^ithont preparation or intermediate forms. Even in the great variations observed by C. C. Adams in the forms of the fresh-water snail lo, jieculiar to the upper tributaries of the Tennessee River above Chattanooga, the variations are evidently due to the varying nature of the bottoms of the streams, the rapidity of the current, and the forced isolation of the varieties, the different forms, from smooth to very spiny, being closely correlated to the varying nature of each stream, and different sections of each stream from the headwaters to the month. In this case there is apparently no action of natural selection, but a direct response to the environment, and the gaps may thus be either marked or slight. In the hundreds of sub-varieties of Helix iiritioralis, the gaps or intervals between such forms are dis- tinct and well marked. (9) Xatural selection is manifestly inadequate to account for the origin of the principal types or classes of plants and animals. They must have appeared with comparative suddenness, as the result of clianges of the conditions of life, inducing new needs, new habits, and the origin by exercise of new organs. Thus the types of co>lenterates, echinoderms, Crustacea, flying in- sects, amphibia, reptiles, birds, and mammals were the result of the action of the changed en- vironment, of effort, use. isolation, and heredity, at a time when the ancestral forms were more plastic than their descendants. (10) Xatural selection, it has been claimed, could not have begun to act until the earth had Vol. XIV.— lu liccome sufficiently well stocked with plant and animal life to afford materials for competition and survival of the fittest. The first forms of life must have arisen through the operation of the Lamareki'an factors. In conclusion it may be said that it still remains an open question whether natural selectiim is an active or by any means universal agent in evolution. Finally it may be observed that the processes of evolution are in kind like those of simple growth of the individual organism, and due to the same factors, and as in ontogenesis there is no one predomi- nant factor, so in phylogenesis there is no one predominant factor, no preponderating mechan- ism such as has been ascribed to natural selec- tion. In the opinion of some expert working naturalists, the greater number of known species have been produced without its aid. It is not of the same nature as artificial selection. Yet the theory is widely accepted, and by its aid Dar- win converted the world to a belief in evolution in general. The views 'widely accepted' as to the relation of natural selection to the other factors of or- ganic evolution may be tabulated thus: I. Primary factors. Direct. — Changes of cosmical environment, changes of climate, light, darkness, tempera- ture, dryness, and humidity, physical and chemical constitution of the soil and of waters, mechanical .state of the miliett, winds, currents of water, biological environment, food, competition, parasitism, symbiosis. Indirect. — P.eaction against cosmical environ- mental conditions: adaptation, convergence, reaction against biological conditions, mim- icry. II. Secondary factors. Heredity, vital concurrence, natural and sexual selection, segregation, geographical isolation, amixia, hybridity. For bibliography, see Evolution. Germinal Selection. This doctrine, founded by Weismann. is an extension of the general idea of natural selection. To 'superorganic' selection, ordinary 'individual' or 'personal' selection, Roux's theory of histological ('histonal') or in- tracellular struggle for existence. Weismann has added the idea of a struggle among the hypo- thetical 'determinants' within the germ, or ger- minal selection. J. A. Thompson has indicated the importance of a form of struggle lying be- tween Roux's histonal selection and Weismann's germinal selection, namely, the struggle between gametes or polential gametes, e.g. between young ova. between sperms, even between ova and sperms. "A vivid realization of this visible struggle." says Thompson, "and the sometimes discriminate selection which it implies, may lead naturally to an appreciation of germinal selec- tion, which deals with the wholly invisible." Al- though opinions may differ as to the existence of this hypothetical phase of selection, the writer jusk quoted claims that the theory "justifies itself provisionally as a formula unifying a large numher of otherwise unrelated facts of inherit- ance." See Veism. nism. NATURAL THEOLOGY. The systematic arrangement of that knowledge as to God and man and their mutual relation? which can be gained from the study of natiire to the exclusion of revelation. Such, at least, is the ideal of