Dimorphism

“ Further, in the fly-catchers we see every gradation of sexual dimorphism, from a difference so slight as to be perceptible only when the sexes are seen side by side, to a difference so great as to make it difficult to believe that the sexes belong to one and the same species. ”

“ Individuals of the same species often present, as is known to every one, great differences of structure, independently of variation, as in the two sexes of various animals, in the two or three castes of sterile females or workers among insects, and in the immature and larval states of many of the lower animals. There are, also, cases of dimorphism and trimorphism, both with animals and plants. ”

“ A second point which appears to me to be elucidated by seasonal dimorphism, is the origin of variability. It has already been prominently shown that secondary forms are for the most part considerably more variable than primary forms. From this it follows that similar external influences either induce different changes in the different individuals of a species, or else change all individuals in the same manner, variability arising only from the unequal time in which the individuals are exposed to the external influence. ”

“ It is in form that we must seek the differences between species, and not in the combinations of matter, which are almost the same in all. [53] The differences are to be sought at the level of the second and third degrees of composition.
The existence of differences of form introduces a new problem, the problem of diversity. There are only a few possible combinations of the principal organs, but as you get down to less important parts the possible scope of variation is greatly increased, and most of the possible variations do exist. ”

“ Any form or condition of structure under a species which differs in its characteristics from those typical to the species, as in color, shape, size, and the like, and which is capable either of perpetuating itself for a period, or of being perpetuated by artificial means; also, any of the various forms under a species meeting the conditions mentioned. A form characterized by an abnormity of structure, or any difference from the type that is not capable of being perpetuated through two or more generations, is not called a variety. ”

“ Bodies, which differ more than in magnitude, are called unlike; and those, which differ only in magnitude, like. Also, of unlike bodies, some are said to differ in the species, others in the genus; in the species, when their difference is perceived by one and the same sense, as white and black; and in the genus, when their difference is not perceived but by divers senses, as white and hot. ”

“ Thus all human beings belong only to one species, which may, however, be divided into five races, and these again into an infinity of varieties. The differences between a race and a variety are, that the latter is a subdivision of the former, and that in the former the modifications are more profound, the changes not being confined to the surface, but extending to the frame-work of the body; whereas, to constitute a variety, nothing more is necessary than the superficial influence of heat and light on the skin, and its appendages the hairs. ”

“ We shall see that they form patterns, considerable in size and of a curious variety of shape, whose boundaries can be firmly outlined, and which are little worlds in themselves. They have the unique merit of retaining all their peculiarities unchanged throughout life, and afford in consequence an incomparably surer criterion of identity than any other bodily feature. They may be made to throw welcome light on some of the most interesting biological questions of the day, such as heredity, symmetry, correlation, and the nature of genera and species. ”

“ We need not be surprised, therefore, that if we next seek to add a third characteristic, say the shape of the head, to this combination of hair and eyes, we find the proportion of pure types combining all three traits in a fixed measure to be very small indeed. Imagine a fourth trait, stature, or a fifth, nose, to be added, and our proportion of pure type becomes almost infinitesimal. ”

“ Beginning with 'Substance,' as summum genus, and adding the difference 'Corporeal,' we frame the species 'Body.' Taking 'Body' as the genus and adding the difference 'Animate,' we frame the species 'Living Body;' and so on till 'Man' is reached; which, being infima species, is only subdivisible into individuals. But the division of Man into individuals involves a change of principle; it is a division of the denotation, not an increase of the connotation as in the earlier steps. ”

“ That part of biological science which deals with form and structure is called Morphology—that which concerns itself with function, Physiology—so that we may conveniently speak of these two senses, or aspects, of "species"—the one as morphological, the other as physiological. Regarded from the former point of view, a species is nothing more than a kind of animal or plant, which is distinctly definable from all others, by certain constant, and not merely sexual, morphological peculiarities. ”

“ In passing through these organs of sense and percipient centres, the like groups of forces are segregated, or separated from the unlike groups of forces; and each such differentiated and integrated series of groups of forces, answering to an external genus or species, constitutes a state of consciousness which we call our idea of the genus or species. We before saw that as well as a separation of mixed matters by the same force, there is a separation of mixed forces by the same matter ”

“ In Morphology we should not speak of Gestalt or fixed form, or if we do we should understand by it only a momentary phase of Bildung. Form is of interest not in itself but only as the manifestation of the inner activity of the living being. Over development, he says elsewhere, there presides a formative force, a bildende Kraft or Bildungstrieb, which works out the idea of the organism. Living things, in his view of them, strive to manifest an idea. They are Nature's works of art—and so, incidentally, they require an artist to interpret them. ”

“ For the transformists, as is well known, variety, race, and species are not fixed concepts. “From variety to race, from race to species, there is a continuous insensible passage. Individual modifications, at first slight, give rise to a variety or to a race. ”

“ It is manifest that man is now subject to much variability. No two individuals of the same race are quite alike. We may compare millions of faces, and each will be distinct. There is an equally great amount of diversity in the proportions and dimensions of the various parts of the body; the length of the legs being one of the most variable points. (1. ‘Investigations in Military and Anthropological Statistics of American Soldiers,’ by B.A. Gould, 1869, p. 256.) ”

“ In this sense the differential element, or differentia specifica, determines the generic element, or genus, of a substance: it tells us what kind or species the substance is: e.g. what kind of animal a man is, viz. rational; what kind of living thing an animal is, viz. sentient; what kind of body or corporeal thing a plant is, viz. living. ”

“ But evolutionary morphology for all practical purposes was a development of pure or idealistic morphology, and was powerless to bring to fruit the new conception with which evolution-theory had enriched it. ”

“ Accordingly, form is one of the least important elements to be considered in determining a homology. "Nature," he wrote in one of his early papers, "tends to repeat the same organs in the same number and in the same relations, and varies to infinity only their form. In accordance with this principle I shall have to draw my conclusions, in the determining the bones of the fish's skull, not from a consideration of their form, but from a consideration of their connections." ”

“ But now, as the genus and the species have no material existence, how can they vary? If individuals alone exist, how can the differences which may be observed among them prove the variability of the species? To which we reply by asking, Which does the question refer to, the category of thought, or the individual embodiment? If the former, then we would remark that our categories of thought vary from time to time in the readiest manner. ”

“ We find ourselves reduced to an arbitrary determination, which sometimes leads us to seize upon the slightest differences of varieties, and form from them the distinctive character of what we call a species, and at other times leads us to consider as a variety of a certain species individuals a little bit different, which others regard as forming a separate species. ”

“ But whatever means Nature adopts, her aim is always the same—to specialise, to differentiate, to produce diversity from uniformity.
Differentiation not only raises the level of organisation; it usually also takes the direction of adaptation to particular habits of life, and this is perhaps the most fruitful cause of diversity. ”

“ Sometimes a term connotes all the attributes that another does, and more besides, which, as distinguishing it, are called differential. Thus 'man' connotes all that 'animal' does, and also (as differentiæ) the erect gait, articulate speech, and other attributes. In such a case as this, where there are well-marked classes, the term whose connotation is included in the others' is called a Genus of that Species. We have a Genus, triangle; and a Species, isosceles, marked off from all other triangles by the differential quality of having two equal sides: again—Genus, book; Species, quarto ”

“ I quite agree with your distinction between dimorphic forms and varieties; but I doubt whether your criterion of dimorphic forms not producing intermediate offspring will suffice, for I know of a good many varieties which must be so called that will not blend or intermix, but produce offspring quite like either parent.
I have been particularly struck with your remarks on geographical distribution in Celebes. It is impossible that anything could be better put, and would give a cold shudder to the immutable naturalists. ”

“ It has been so generally the practice to speak of variation as something exceptional and comparatively rare—as an abnormal deviation from the uniformity and stability of the characters of a species—and so few even among naturalists have ever compared, accurately, considerable numbers of individuals, that the conception of variability as a general characteristic of all dominant and widespread species, large in its amount and affecting, not a few, but considerable masses of the individuals which make up the species, will be to many entirely new. ”

“ Though the theory of natural selection took little account of the problems of form, Darwin's masterly vindication of the theory of evolution was of immense service to morphology, and Darwin himself was the first to point out what a great light evolution threw upon all morphological problems. ”

“ Hence modifications of structure, viewed by systematists as of high value, may be wholly due to the laws of variation and correlation, without being, as far as we can judge, of the slightest service to the species. ”

“ Morphology considers not only forms, but it also considers kinds, forces, causes, and concepts. Dynamics considers not only forces, but it also considers kind, forms, causes, and concepts. Evolution considers not only causes, but it considers kinds, forms, forces, and concepts; and ideation considers not only concepts, but it also considers kinds, forms, forces, and causes, and the difference between these five concomitants is the point of view when every one of the essentials and its derivatives is considered abstractly. ”

“ To all which you may add, that the least imaginable variation, either in the terms or the structure of them, not only destroys the identity, but often disfigures the resemblance to that degree that we hardly know it to be a resemblance. ”

“ Whatever may be the explanation suggested by one or another hypothesis as to the mode in which this uniformity of structure came to exist, it is certain that it does exist. Twenty segments are found in hundreds of thousands of species which are immensely different from each other in size, outline, substance and modes of existence. ”

“ A globe, the most uniform of all figures, is of all the most beautiful; and a square, though more beautiful than a trapezium, hath less variety in its constituent parts. The foregoing definition, which at best is but obscurely expressed, is only applicable to a number of objects in a group or in succession, among which indeed a due mixture of uniformity and variety is always agreeable, provided the particular objects, separately considered, be in any degree beautiful. Uniformity amidst variety among ugly objects, affords no pleasure. ”

“ The close affinity of transcendentalists and evolutionists is shown very clearly in their common contrast in habits of 305thought with the Cuvierian school. It is the cardinal principle of pure morphology that function must be excluded from consideration. This is a necessary and unavoidable simplification which must be carried out if there is to be a science of pure form at all. But this limitation of outlook, if carried over from morphology to general biology becomes harmful, since it wilfully ignores one whole side of life—and that the most important. ”

“ The amount of difference between the several groups—that is the amount of modification which each has undergone—will be expressed by such terms as genera, families, orders, and classes. As we have no record of the lines of descent, these lines can be discovered only by observing the degrees of resemblance between the beings which are to be classed. For this object numerous points of resemblance are of much more importance than the amount of similarity or dissimilarity in a few points. ”

“ While the number of factors involved in a morphological problem is vastly greater than that of those which bear upon any problem into which life does not enter, and while the relations between these factors vary, in closely allied cases, in a way which has no parallel outside life-science, the problems of general morphology are still vastly simpler than those of society or of morality, or of almost any other department of human conduct, although they are like them in kind, and supply the same sort of evidence. ”