(S) Since Darwin called attention to the lack of long series of intermediate links between spe cies, naturalists have been more and more in clined to the belief that such series of connect ing variations have never existed, but that na ttn•e makes leaps, that species oft el 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, W. H. Dall, (Talton, De Vries, and many others, have advo cated the view of the rapid or sudden modifica tion of structure involving the sudden appear ance of species.. especially in Paleozoic times. This is seen by the results of the examination of the Steinheim 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 Vries originated suddenly, without preparation o• intermediate forms. Even in the great variations observed by C. C. Adams in the forms of the fresh-water snail To, peculiar to the upper tributaries of the Tennessee Rive• alcove Chattanooga. the variations are evidently due to the varying nature of the bottoms of the streams, the rapidity of the cm-rent, 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 mouth. 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 o• slight. In the hundreds of sub-varieties of Helix nemoralis, the gaps or intervals between such forms are dis tinct and well marked.
(9) Natural 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 changes of the conditions of life, inducing new needs, new habits• and the origin by exercise of new organs. Thus the types of
caelente•ates, echinoderms, crustacen. flying in sects, amphihia. 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 deseendants.
(10) Natural selection, it has been claimed. could not have begun to act until the earth had become 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 Lanmrekian factors. En conehlaion it may be said that it still remains an open question whether natural selection is an active o• 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 outogenesis 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 sonic 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 co-mica! 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 milieu, winds, currents of water, biological environment. food. competition, parasitism. symbiosis.
bidirect.—Reaction 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.