In an essay ent.i.tled "The Ancestry of Insects"[219] (1873) we adopted the Lamarckian factors of change of habits and environment, of use and disuse, to account for the origin of the appendages, while we attributed the origin of the metamorphoses of insects to change of habits or of the temperature of the seasons and of climates, particularly the change in the earth"s climates from the earlier ages of the globe, "when the temperature of the earth was nearly the same the world over, to the times of the present distribution of heat and cold in zones."
From further studies on cave animals, published in 1877,[220] we wrote as follows:
"In the production of these cave species, the exceptional phenomena of darkness, want of sufficient food, and unvarying temperature, have been plainly enough _verae causae_. To say that the principle of natural selection accounts for the change of structure is no explanation of the phenomena; the phrase has to the mind of the writer no meaning in connection with the production of these cave forms, and has as little meaning in accounting for the origination of species and genera in general. Darwin"s phrase "natural selection," or Herbert Spencer"s term "survival of the fittest,"
expresses simply the final result, while the process of the origination of the new forms which have survived, or been selected by nature, is to be explained by the action of the physical environments of the animals coupled with inheritance-force. It has always appeared to the writer that the phrases quoted above have been misused to state the cause, when they simply express the result of the action of a chain of causes which we may, with Herbert Spencer, call the "environment" of the organism undergoing modification; and thus a form of Lamarckianism, greatly modified by recent scientific discoveries, seems to meet most of the difficulties which arise in accounting for the origination of species and higher groups of organisms. Certainly "natural selection" or the "survival of the fittest" is not a _vera causa_, though the "struggle for existence" may show us the causes which have led to the _preservation_ of species, while changes in the environment of the organism may satisfactorily account for the original tendency to variation a.s.sumed by Mr. Darwin as the starting-point where natural selection begins to act."
In our work on _The Cave Animals of North America_,[221] after stating that Darwin in his _Origin of Species_ attributed the loss of eyes "wholly to disuse," remarking (p. 142) that after the more or less perfect obliteration of the eyes, "natural selection will often have effected other changes, such as an increase in the length of the antennae or palpi, as a compensation for blindness," we then summed up as follows the causes of the production of cave faunae in general:
"1. Change in environment from light, even partial, to twilight or total darkness, and involving diminution of food, and compensation for the loss of certain organs by the hypertrophy of others.
"2. Disuse of certain organs.
"3. Adaptation, enabling the more plastic forms to survive and perpetuate their stock.
"4. Isolation, preventing intercrossing with out-of-door forms, thus insuring the permanency of the new varieties, species, or genera.
"5. Heredity, operating to secure for the future the permanence of the newly originated forms as long as the physical conditions remain the same.
"Natural selection perhaps expresses the total result of the working of these five factors rather than being an efficient cause in itself, or at least const.i.tutes the last term in a series of causes.
Hence Lamarckism in a modern form, or as we have termed it, Neolamarckism, seems to us to be nearer the truth than Darwinism proper or natural selection."[222]
In an attempt to apply Lamarck"s principle of the origin of the spines and horns of caterpillars and other insects as well as other animals to the result of external stimuli,[223] we had not then read what he says on the subject. (See p. 316.) Having, however, been led to examine into the matter, from the views held by recent observers, especially Henslow, and it appearing that Lamarck was substantially correct in supposing that the blood (his "fluids") would flow to parts on the exposed portions of the body and thus cause the origin of horns, on the principle of the saying, "_ubi irritatio, ibi affluxus_," we came to the following conclusions:
"The Lamarckian factors (1) change (both direct and indirect) in the _milieu_, (2) need, and (3) habit, and the now generally adopted principle that a change of function induces change in organs,[224]
and in some or many cases actually induces the hypertrophy and specialization of what otherwise would be indifferent parts or organs;--these factors are all-important in the evolution of the colors, ornaments, and outgrowths from the cuticle of caterpillars."
Our present views as to the relations between the Lamarckian factors and the Darwinian one of natural selection are shown by the following summary at the end of this essay.
"1. The more prominent tubercles, and spines or bristles arising from them, are hypertrophied piliferous warts, the warts, with the seta or hair which they bear, being common to all caterpillars.
"2. The hypertrophy or enlargement was probably [we should rather say _possibly_] primarily due to a change of station from herbs to trees, involving better air, a more equable temperature, perhaps a different and better food.
"3. The enlarged and specialized tubercles developed more rapidly on certain segments than on others, especially the more prominent segments, because the nutritive fluids would tend more freely to supply parts most exposed to external stimuli.
"4. The stimuli were in great part due to the visits of insects and birds, resulting in a mimicry of the spines and projections on the trees; the colors (lines and spots) were due to light or shade, with the general result of protective mimicry, or adaptation to tree-life.
"5. As the result of some unknown factor some of the hypodermic cells at the base of the spines became in certain forms specialized so as to secrete a poisonous fluid.
"6. After such primitive forms, members of different families, had become established on trees, a process of arboreal segregation or isolation would set in, and intercrossing with low-feeders would cease.
"7. Heredity, or the unknown factors of which heredity is the result, would go on uninterruptedly, the result being a succession of generations perfectly adapted to arboreal life.
"8. Finally the conservative agency of natural selection operates constantly, tending towards the preservation of the new varieties, species, and genera, and would not cease to act, in a given direction, so long as the environment remained the same.
"9. Thus in order to account for the origin of a species, genus, family, order, or even a cla.s.s, the first steps, causing the origination of variations, were in the beginning due to the primary (direct and indirect) factors of evolution (Neolamarckism), and the final stages were due to the secondary factors, segregation and natural selection (Darwinism)."
From a late essay[225] we take the following extracts explaining our views:
"In seeking to explain the causes of a metamorphosis in animals, one is compelled to go back to the primary factors of organic evolution, such as the change of environment, whether the factors be cosmical (gravity), physical changes in temperature, effects of increased or diminished light and shade, under- or over-nutrition, and the changes resulting from the presence or absence of enemies, or from isolation. The action of these factors, whether direct or indirect, is obvious, when we try to explain the origin or causes of the more marked metamorphoses of animals. Then come in the other Lamarckian factors of use and disuse, new needs resulting in new modes of life, habits, or functions, which bring about the origination, development, and perfection of new organs, as in new species and genera, etc., or which in metamorphic forms may result in a greater increase in the number of, and an exaggeration of the features characterizing the stages of larval life.
"VI. _The Adequacy of Neolamarckism_.
"It is not to be denied that in many instances all through the ceaseless operation of these fundamental factors there is going on a process of sifting or of selection of forms best adapted to their surroundings, and best fitted to survive, but this factor, though important, is quite subordinate to the initial causes of variation, and of metamorphic changes.
"Neolamarckism,[226] as we understand this doctrine, has for its foundation a combination of the factors suggested by the Buffon and Geoffroy St. Hilaire school, which insisted on the direct action of the _milieu_, and of Lamarck, who relied both on the direct (plants and lowest animals) and on the indirect action of the environment, adding the important factors of need and of change of habits resulting either in the atrophy or in the development of organs by disuse or use, with the addition of the hereditary transmission of characters acquired in the lifetime of the individual.
"Lamarck"s views, owing to the early date of his work, which was published in 1809, before the foundation of the sciences of embryology, cytology, palaeontology, zoogeography, and in short all that distinguishes modern biology, were necessarily somewhat crude, though the fundamental factors he suggested are those still invoked by all thinkers of Lamarckian tendencies.
"Neolamarckism gathers up and makes use of the factors both of the St. Hilaire and Lamarckian schools, as containing the more fundamental causes of variation, and adds those of geographical isolation or segregation (Wagner and Gulick), the effects of gravity, the effects of currents of air and of water, of fixed or sedentary as opposed to active modes of life, the results of strains and impacts (Ryder, Cope, and Osborn), the principle of change of function as inducing the formation of new structures (Dohrn), the effects of parasitism, commensalism, and of symbiosis--in short, the biological environment; together with geological extinction, natural and s.e.xual selection, and hybridity.
"It is to be observed that the Neolamarckian in relying mainly on these factors does not overlook the value of natural selection as a guiding principle, and which began to act as soon as the world became stocked with the initial forms of life, but he simply seeks to a.s.sign this principle to its proper position in the hierarchy of factors.
"Natural selection, as the writer from the first has insisted, is not a _vera causa_, an initial or impelling cause in the origination of new species and genera. It does not start the ball in motion; it only, so to speak, guides its movements down this or that incline.
It is the expression, like that of "the survival of the fittest" of Herbert Spencer, of the results of the combined operation of the more fundamental factors. In certain cases we cannot see any room for its action; in some others we cannot at present explain the origin of species in any other way. Its action increased in proportion as the world became more and more crowded with diverse forms, and when the struggle for existence had become more unceasing and intense. It certainly cannot account for the origination of the different branches, cla.s.ses, or orders of organized beings. It in the main simply corresponds to artificial selection; in the latter case, man selects forms already produced by domestication, the latter affording sports and varieties due to change in the surroundings, that is, soil, climate, food, and other physical features, as well as education.
"In the case also of heredity, which began to operate as soon as the earliest life forms appeared, we have at the outset to invoke the principle of the heredity of characters acquired during the lifetime of lowest organisms.
"Finally, it is noticeable that when one is overmastered by the dogma of natural selection he is apt, perhaps unconsciously, to give up all effort to work out the factors of evolution, or to seek to work out this or that cause of variation. Trusting too implicitly to the supposed _vera causa_, one may close his eyes to the effects of change of environment or to the necessity of constant attempts to discover the real cause of this or that variation, the reduction or increase in size of this or that organ; or become insensible to the value of experiments. Were the dogma of natural selection to become universally accepted, further progress would cease, and biology would tend to relapse into a stage of atrophy and degeneration. On the other hand, a revival of Lamarckism in its modern form, and a critical and doubting att.i.tude towards natural selection as an efficient cause, will keep alive discussion and investigation, and especially, if resort be had to experimentation, will carry up to a higher plane the status of philosophical biology."
Although now the leader of the Neodarwinians, and fully a.s.sured of the "all-sufficiency" of natural selection, the veteran biologist Weismann, whose earlier works were such epoch-making contributions to insect embryology, was, when active as an investigator, a strong advocate of the Lamarckian factors. In his masterly work, _Studies in the Theory of Descent_[227] (1875), although accepting Darwin"s principle of natural selection, he also relied on "the transforming influence of direct action as upheld by Lamarck," although he adds, "its extent cannot as yet be estimated with any certainty." He concluded from his studies in seasonal dimorphism, "that differences of specific value can originate through the direct action of external conditions of life only." While conceding that s.e.xual selection plays a very important part in the markings and coloring of b.u.t.terflies, he adds "that a change produced directly by climate may be still further increased by s.e.xual selection."
He also inquired into the origin of variability, and held that it can be elucidated by seasonal dimorphism. He thus formulated the chief results of his investigations: "A species is only caused to change through the influence of changing external conditions of life, this change being in a fixed direction which entirely depends on the physical nature of the varying organism, and is different in different species or even in the two s.e.xes of the same species."
The influence of changes of climate on variation has been studied to especial advantage in North America, owing to its great extent, and to the fact that its territory ranges from the polar to the tropical regions, and from the Atlantic to the Pacific Ocean. As respects climatic variation in birds, Professor Baird first took up the inquiry, which was greatly extended, with especial relation to the formation of local varieties, by Dr. J. A. Allen,[228] who was the first to ascertain by careful measurements, and by a study of the difference in plumage and pelage of individuals inhabiting distant portions of a common habitat, the variations due to climatic and local causes.
"That varieties," he says, "may and do arise by the action of climatic influences, and pa.s.s on to become species; and that species become, in like manner, differentiated into genera, is abundantly indicated by the facts of geographical distribution, and the obvious relation of local forms to the conditions of environment. The present more or less unstable condition of the circ.u.mstances surrounding organic beings, together with the known mutations of climate our planet has undergone in past geological ages, point clearly to the agency of physical conditions as one of the chief factors in the evolution of new forms of life. So long as the environing conditions remain stable, just so long will permanency of character be maintained; but let changes occur, however gradual or minute, and differentiations begin." He inclines to regard the modifications as due rather to the direct action of the conditions of environment than to "the round-about process of natural selection."
He also admits that change of habits and food, use and disuse, are factors.
The same kind of inquiry, though on far less complete data, was extended by the present writer[229] in 1873 to the moths, careful measurements of twenty-five species of geometrid moths common to the Atlantic and Pacific coasts of North America showing that there is an increase in size and variation in shape of the wings, and in some cases in color, in the Pacific Coast over Eastern or Atlantic Coast individuals of the same species, the differences being attributed to the action of climatic causes. The same law holds good in the few Notodontian moths common to both sides of our continent. Similar studies, the results depending on careful measurements of many individuals, have recently been made by C. H. Eigenmann (1895-96), W. J. Moenkhaus (1896), and H. C. b.u.mpus (1896-98).
The discoveries of Owen, Gaudry, Huxley, Kowalevsky, Cope, Marsh, Filhol, Osborn, Scott, Wortmann, and many others, abundantly prove that the lines of vertebrate descent must have been the result of the action of the primary factors of organic evolution, including the principles of migration, isolation, and compet.i.tion; the selective principle being secondary and preservative rather than originative.
Important contributions to dynamic evolution or kinetogenesis are the essays of Cope, Ryder, Dall, Osborn, Jackson, Scott, and Wortmann.
Ryder began in 1877 to publish a series of remarkably suggestive essays on the "mechanical genesis," through strains, of the vertebrate limbs and teeth, including the causes of the reduction of digits. In discussing the origin of the great development of the incisor teeth of rodents, he suggested that "the more severe strains to which they were subjected by enforced or intelligently a.s.sumed changes of habit, were the initiatory agents in causing them to a.s.sume their present forms, such forms as were best adapted to resist the greatest strains without breaking."[230]
He afterwards[231] claimed that the articulations of the cartilaginous fin-rays of the trout (_Salmo fontinalis_) are due to the mechanical strains experienced by the rays in use as motors of the body of the fish in the water.
In the line of inquiry opened up by Cope and by Ryder are the essays of Osborn[232] on the mechanical causes for the displacement of the elements of the feet in the mammals, and the phylogeny of the teeth.
Also Professor W. B. Scott thus expresses the results of his studies:[233]
"To sum up the results of our examination of certain series of fossil mammals, one sees clearly that transformation, whether in the way of the addition of new parts or the reduction of those already present, acts just _as if_ the direct action of the environment and the habits of the animal were the efficient cause of the change, and any explanation which excludes the direct action of such agencies is confronted by the difficulty of an immense number of the most striking coincidences.... So far as I can see, the theory of determinate variations and of use-inheritance is not antagonistic but supplementary to natural selection, the latter theory attempting no explanation of the _causes_ of variation. Nor is it pretended for a moment that use and disuse are the sole or even the chief factors in variation."
As early as 1868 the Lamarckian factor of isolation, due to migration into new regions, was greatly extended, and shown by Moritz Wagner[234]
to be a most important agent in the limitation and fixation of varieties and species.
"Darwin"s work," he says, "neither satisfactorily explains the external cause which gives the first impulse to increased individual variability, and consequently to natural selection, nor that condition which, in connection with a certain advantage in the struggle for life, renders the new characteristics indispensable.
The latter is, according to my conviction, solely fulfilled by the voluntary or pa.s.sive migration of organisms and colonization, which depends in a great measure upon the configuration of the country; so that only under favorable conditions would the home of a new species be founded."