TYPEN DER ARTBILDUNG

Abstract

SummaryUndirected mutation and natural selection may be regarded as sufficient premises for evolution. In view of the manifold effects of mutations there is no justification in'assuming an inherent process of unfolding. There is no simple scheme of species formation, as has been assumed by some geneticists, but there exist various types of race and species development corresppnding to the complex conditions of existence of the different organisms. The following cases can be distinguished:1. Undirected mutation leading to new races and species, without recognizable selection (e.g. Rhipidura flabellifera). Alterations of phylogenetically important organs, for example, genital organs, may be involved.2. Race and species formation taking place in definite directions, without selection being recognizable (e.g. snails of the Murella group).3. Selection occurring in definite directions. Through selection by climatic factors, this may lead to the parallel evolution of characters in different forms inhabiting the same region, which can be defined by various biological rules (e.g. geographical races of Acro‐cephalus arundinaceus).4. The causes of transformations of the body as a whole cannot easily be recognized, but these are likewise explicable by undirected mutation and selection:(a) Orthogenetic series arise mostly through continual selection of the larger individual variants and the resulting changes of proportions due to the heterogonic growth of single organs, or through orthoselection (e.g. the ancestral series of Equrcs).(b) Insufficient attention has hitherto been paid to phenomena of compensation of growing body material. In this article they have been considered in greater detail (e.g. spines of the fresh‐water gastropod Melania). They can explain not only transformations of the body as a whole, but also reductions and the formation of rudimentary stractures.(c) Alterations of anatomical structure can be understood without an active reaction of the genes to environmental changes (e.g. skulls of Fotamochoerrcs, Hylochoerus, Phaco‐choerus).(d) More emphasis should be placed on the pleiotropic action of most, or perhaps all, genes, i.e. the alteration of several characters by one mutation. This is particularly important in systematic studies (e.g. body size, tail, ears and feet of Muridae).5. The evolutionary phenomena recognized in palaeontology can be understood without the assumption of any inner unfolding impulse:(a) Although the irreversibility of evolution has been demonstrated in fact in numerous cases, it has been dispiended theoretically by the discovery of back‐mutations. The actual irreversibility is due to the complex conditions of life which render a return of exactly the same situations for mutation and selection practically impossible.(b) Orthogenetic series have been dealt with under 4 (a).(c) Explosive development of new forms and later diminution of species formation should not be ascribed to varying rates of mutation, since mutation is steady, but to increasing occupation of the available biotopes with competing forms. Ageing and extinction of evolutionary series is to be explained by an increased restriction to narrow habitats and specialization due to selection, by selectively caused increase in size, and by the consequent development of monstrous organs.(d) Even the perfection and the higher development of organisms is due to no unknown law of development, but merely to the fulfilment of the available possibilities, including increase in complication and physiological amelioration and simplification.