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sure; ascent is not up a single ladder or along a single genealogical branch, but these branches are few (as indeed we actually know them to be, however the restriction may be brought about) and the evolution is always progressive, that is, toward what we, from an anthropocentric point of view, are constrained to call higher or more ideal life stages and conditions.
Other naturalists also seeming to see this course of determinate or orthogenetic evolution, but not inclined to surrender their disbelief in vitalism, in forces over and beyond the familiar ones of the physicochemical world, have tried to adduce a definite causomechanical explanation of orthogenesis. The best and most comprehensible types of this explanation are those essentially Lamarckian in principle, in which the direct influence on living matter of environmental conditions, the direct reactions of the life stuff to stimuli and influences from the world outside, are the causal factors in such an explanation. But while every naturalist will grant that such factors do change and control in considerable degree the life of the individual, most see no mechanism or means of extending this control directly to the species.
The stumbling block of heredity, the means and mode of inheritance, as we so far know them, are directly in the way of any general acceptance of such a theory of evolution under the direct control of such “primary factors of life.” Ontogenetic species, that is, conditions of structure and habit common to many individuals of one kind, the conditions due to sameness of intrinsic and extrinsic factors in development, constitute a category of organisms which at any given time and place scem very real, and are for the moment truly real. But their environment is remaining fairly constant. We speak easily of the flux of Nature: her everchangingness. And in the large we are speaking only of the truth. But during our brief period of observation of the few generations of this or that kind of animal or plant that come under our eyes and microscopes, the nature environing these generations may be nearly uniform. What are the changes in the desert in a score or a hundred or a thousand of years? What changes in life conditions on the barren storm-swept peaks of the mountain ranges? What in the waters of that brackish bay or sweet-water lake apart from the paths of man? Ontogenetic species have a seeming of reality, but so far as our present knowledge goes it is only a seeming: reality vanishes with the death of the individual: their young can perpetuate their specific peculiarities only if the environmental conditions of their development are identical with those which attended the growing up of their parents. Variations in this environment will determine variations in them, and their father's kind will exist no more.
The authors of this book believe that more characters of species than are commonly thought are of this shifty, ephemeral character; that not a few so-called true species are only ontogenetic species held for a number of generations true to a type simply because the environment, the extrinsic factors in the development of all the individuals in these successive gencrations, are the same. But how these individual characteristics and changes can be put into the heredity of the race we do not understand. “There is no fixity in species other than that due to the long-repeated ontogenetic reiteration of this or that characteristic,” says Luther Burbank. And he speaks from the conviction forced on him through thirty years of the closest sort of observation and personal experience of the life of plants. And yet, however strongly our own minds respond to a desire to believe this-it would be so clarifying—the obstinate “no mechanism” objection stands boldly up to check our sympathetic reasoning.
Finally we should refer to the theories of heterogenesis or species-forming by mutations or saltations, which have been proposed at various times as a substitute for the theory of species-forming by the gradual transformation through selection. During the discussion in the first few years after the appearance of Darwin's “Origin of Species," the German zoologist von Kölliker expressed the belief that the change from species to species would probably be found to be more sudden and more distinctive than Darwin's theory permitted one to assume. Later, the Russian botanist Korschinsky, on a basis of general observation and some not very extensive personal experimentation, definitely formulated a theory of speciesforming by heterogenesis which he placed strongly in contrast with Darwin's theory of gradual transformation by selection, which later theory he claimed should be wholly given up. But not until the publication of de Vries's work, Die Mutationstheorie, in which are recorded the results of close personal observation and experimentation for twenty years on race and
species-forming in plants has the theory of species-forming by mutations, or sudden fixed changes (lesser or greater) had any considerable adoption or even general attention.
At the present moment, probably because of a strong reaction against the too blind acceptance and general overemphasis of the selection doctrines, and because, too, of the unusually extensive character of de Vries's experimentation and observation, and his trenchant criticism of the weak places in the other theories, with the generally weighty character of his work and reputation, because of all this the theory of speciesforming by mutations has at the present moment a fairly large body of adherents among reputable biologists. And yet the actual evidence of tested observation on which the theory rests is curiously meager. One hastens to admit, however, that similar evidence for the theory of direct species-forming by selection is also meager. While apparently no one has ever seen a case of species-making by the natural selection of slight fluctuating variations, de Vries seems to be almost the only one who has observed actual cases of species-making by heterogenesis, and he has seen very few. And in the nature of things, the opportunities for this kind of evidence, that is, that of actual observation, ought to be much larger in the case of heterogenesis than in that of general transformation by the selection of slight variations. An account of the exact character of the de Vriesian mutations is included in our later chapter on variation and mutation. Our readers should realize, that however much they may see of this theory in present-day popular scientific literature, and however strongly the case may be put in favor of the mutation theory of species origin, this theory is not accepted by the great body of biologists as entitled to replace the Darwinian theory.
We may close this chapter with a reference to a pregnant sentence of the American paleontologist, Osborn, in a lecture entitled “The Unknown Factors of Evolution”: “The general conclusion we reach from a survey of the whole field is that for Buffon's and Lamarck's factors we have no theory of heredity, while the original Darwinian factor, or Neo-Darwinism, offers an inadequate explanation of evolution. If acquired variations are transmitted, there must be, therefore, some unknown principle in heredity; if they are not transmitted, there must be some unknown factor in evolution.” Our present plight seems to be exactly this: we cannot explain to any general satisfaction species-forming and evolution without the help of some Lamarckian or Eimerian factor; and on the other hand, we cannot assume the actuality of any such factor in the light of our present knowledge of heredity. The discovery of the “unknown factors of evolution" should be the chief goal of all present-day biologic investigation.
GEOGRAPHIC ISOLATION AND SPECIES
“For me, it is the chorology of organisms, the study of all the important phenomena embraced in the geography of animals and plants, which is the surest guide to the knowledge of the real phases in the process of the formation of species.”—MORITZ WAGNER.
A FLOOD of light may be thrown on the general problem of the origin of species by the study of certain evidence as to the l actual origin of species with which we may be familiar, or of which the actual history or the actual ramifications may in some degree be traced.
In such cases, one of the first questions naturally asked is this: Where did the species come from? Migration forms a large part of the history of any species or group of forms. The fauna of any given region is made up of the various species of animals living naturally within its borders. The flora of a region is made up of the plants which grow naturally within its limits. Of all these, animals and plants, the inhabitants of most regions are apparently largely migrants from some other region. Some have entered the region in question before acquiring their present specific characters; others come after having done so. Which of these conditions apply to any given case can sometimes be ascertained by the comparison of the individuals along the supposed route of migration.
Thus, Dr. C. Hart Merriam has undertaken to show the actual origin of nine species of Californian chipmunks (Eutamias) by an elaborate study of their distribution, adaptations, and transformations. He finds them closely related to one another,
"A paper published in “Science,” 1906, by the senior author, under the title “ The Actual Origin of Species," has been freely quoted from in this chapter.