Imagens das páginas
PDF
ePub

CHAPTER VII

VARIOUS THEORIES OF SPECIES-FORMING

AND DESCENT CONTROL

The four factors named, variation, inheritance, selection, and separation, must work together in order to form different species. It is impossible to think that one of these should work by itself or that one could be left aside. -ORTMANN.

As mentioned in the introductory chapter on the factors of evolution (Chapter IV), and as referred to several times in the chapter on natural selection, the factor of the segregation or isolation of groups of individuals must be taken into account in any discussion of species-forming causes. This factor has long been recognized by biologists, that phase of it, and undoubtedly the most important of its several phases, called geographic or topographic isolation or segregation being very clearly stated and its importance emphasized by Moritz Wagner in 1868. Alfred Russel Wallace gave much attention, in his years of active investigation, to the general subject of geographical distribution, and was a pioneer in calling the attention of naturalists to the great significance, in the light of the evolution theory, of the facts of the geographical distribution of both animals and plants. To-day, especially among American biologists, the factor of topographic segregation is recognized as one of the most important of species-molding influences. Indeed it seems self-evident to many naturalists that natural selection is impotent as an actual cause of species-forming without some effective sort of isolation factor to assist it. Because of the importance in the eyes of present-day naturalists of the geographic isolation factor we have given (Chapter VIII), a brief special discussion of this factor. In addition, in Chapter XIV, will be found a discussion of the more general subject of geographical distribution.

108

But it is conceivable that isolation may be effected in other ways than by actual segregation or geographic separation of individuals. Anything that could lead to exclusive or discriminate breeding among certain individuals of a species would result in the isolation of these individuals from the rest of the species as effectively as their actual separation from others by a geographic or topographic barrier. Now there are various influences or conditions that might conceivably bring about such a state of affairs, and some of these have been actually observed to exist. It is of interest to note that this kind of isolation differs, in a rather important way, from purely geographic isolation in that the latter is almost sure to be wholly indiscriminate as regards the individuals comprised in an isolated group, while the former, which has been called physiological isolation, will be discriminate. That is, there will be a structural or physiological peculiarity common to all the “isolated” individuals, it being by virtue of this common peculiarity (something not common to other individuals of the same species) that the isolation actually exists.

Romanes has been the chief champion of the physiological isolation factor. And we may advantageously refer directly to his writings for a specific statement of different forms or phases of this kind of isolation. In “Darwin and After Darwin,” III, p. 7 et seq., he writes:

“Now the forms of discriminate isolation, or homogamy, are very numerous. When, for example, any section of a species adopts somewhat different habits of life, or occupies a somewhat different station in the economy of nature, homogamy arises within that section. There are forms of homogamy on which Darwin has laid great stress, as we shall presently find. Again, when for these or any other reasons a section of a species becomes in any small degree modified as to form or color, if the species happens to be one where any psychological preference in pairing can be exercised—as is very generally the case among the higher animals—exclusive breeding is apt to ensue as a resuit of such preference; for there is abundant evidence to show that, both in birds and mammals, sexual selection is usually opposed to the intercrossing of dissimilar varieties. Once more, in the case of plants, intercrossing of dissimilar varieties may be prevented by any slight difference in their seasons of flowering, of topographical stations, or even, in the case of flowers which depend on insects for their ferti

lization, by differences in the instincts and preferences of their visitors.

“But, without at present going into detail with regard to these different forms of discriminate isolation, there are still two others, both of which are of much greater importance than any that I have hitherto named. Indeed, these two forms are of such immeasurable importance that were it not for their virtually ubiquitous operation, the process of organic evolution could never have begun, nor, having begun, continued.

“The first of these two forms is sexual incompatibility-either partial or absolute-between different taxonomic groups. If all hares and rabbits, for example, were as fertile with one another as they are within their own respective species, there can be no doubt that sooner or later, and on common areas, the two types would fuse into one. And similarly, if the bar of sterility could be thrown down as between all the species of a genus, or all the genera of a family, not otherwise prevented from intercrossing, in time all such species, or all such genera, would become blended into a single type. As a matter of fact, complete fertility, both of first crosses and of their resulting hybrids, is rare, even as between species of the same genus; while as between genera of the same family complete fertility does not appear ever to occur, and, of course, the same applies to all the higher taxonomic divisions. On the other hand, some degree of infertility is not unusual as between different varieties of the same species; and, wherever this is the case, it must clearly aid the further differentiation of those varieties. It will be my endeavor to show that in this latter connection sexual incompatibility must be held to have taken an immensely important part in the differentiation of varieties into species. But meanwhile we have only to observe that wherever such incompatibility is concerned, it is to be regarded as an isolating agency of the very first importance. And as it is of a character purely physiological, I have assigned to it the name Physiological Isolation; while for the particular case where this general principle is concerned in the origination of specific types, I have reserved the name Physiological Selection."

If the factors of variation, heredity, natural selection, and isolation are, in the minds of most naturalists, the chief factors in species-forming and descent control, and a combination of these factors is, in the belief of these same naturalists-the socalled selectionists or Neo-Darwinians—a sufficient causal explanation of organic evolution, there are many other natural

ists who have no such high esteem of the value of natural selection. These believe, variously, that (a) to the selection factors other auxiliary or helping ones are to be added, or (b) that various other factors are equally potent in species-forming, or (c) that these other factors are the more important ones, or finally (d) that the selection factors are of no importance at all, that is, have no reality. Before Darwin, the French naturalist Lamarck had clearly enunciated an explaining theory of species transformation, and there are to-day many naturalists who believe that the Lamarckian explanation, or its fundamental assumption, is true, or, at least, that it is based on the more important and effective factors in evolution. These naturalists have been called Neo-Lamarckians. Some of these have formulated theories of their own based on Lamarckian fundamentals, but developed in directions more or less obviously away from characteristic Lamarckism. Still other fundamental causal factors than the Darwinian ones of selection and the Lamarckian ones of accumulated effect of use, disuse, and functional stimulation are assumed in certain other theories of species change and general evolution. Nägeli, a botanist and natural philosopher, believed in a special inherent vitalistic principle or force in living matter which tends to produce progressive differentiation and evolution. Von Kölliker, Korschinsky, and de Vries believe that species-forming occurs by definite sudden small (or larger) fixed changes or mutations, so that for them a mutational or discontinuous variation is the fundamental causal factor in species transformation. Numerous paleontologists believe that variation follows determinate lines in its occurrence, so that evolution is orthogenetic, with its lines primarily fixed by determinate variation. We may then examine briefly some of the more important special theories or groups of theories put forward by biologists either as auxiliary and subordinate to the more generally known Darwinian theory, or as alternative with or substitutes for this theory. First to be mentioned should be the transmutation theory of Lamarck. In its simplest expression it is, that every individual organism is, throughout its lifetime, reacting to environmental stimuli and conditions in such ways as to change its structure and its habits in greater or less degree from the structure and habits of its parents and ancestors, this change coming about specifically from the varying effects of use or disuse of parts, and the functional stimulation of other parts in response to such extrinsic conditions as light, contact, temperature, pressure, color, etc., etc. The changes effected will, in the nature of things, be essentially adaptive. Now, these adaptive changes, these variations, or new characters acquired during the lifetime of the individual will be, in Lamarck's belief, inherited, if not in full, at least in partial degree, by the offspring. These in turn submitted to similar or to different environmental influences will continue the changes either cumulatively or diversely. By this steady direct change and adaptation to environment the species is ever modifying and transforming. Evolution marches, and marches adaptively and advantageously. But modern naturalists find a most unfortunate impediment to this simple, direct, and sufficient explanation of speciesforming and evolution in the apparent untruth of the assumption that the characters acquired by an individual in its lifetime are transmitted by inheritance to its young. This question, fundamental to the Lamarckian theory, of the inheritance or noninheritance of acquired characters has long been one of the most hotly debated points in evolution biology. As we have devoted a number of pages to its particular discussion in our later chapter on heredity (Chapter X), we need not anticipate that discussion here. It is sufficient to say that as far as scientific proof, that is, evidence fron, actual observation and experiment, goes, those naturalists led by Weismann, who deny this inheritance, have at present distinctly the better of the argument. The orthogenetic evolution theories of various authors, based upon the assumed occurrence of variations in determinate lines or directions (a restricted and determinate variation as compared with the nearly infinite, fortuitous, and indeterminate variation assumed in the selection theory), are of several types. The mention of two will reveal pretty well the more important characters of all. Not a few biologists have always believed in the existence of a sort of mystic, special vitalistic force or principle by virtue of which determination and general progress of evolution is chiefly fixed. Such a capacity, inherent in living matter, seems to include at once possibility of specific adaptation and the possibility of progressive or truly evolutionary change. Not all evolution is in a single direct line, to be

« AnteriorContinuar »