mon characters of the individuals which compose the species. Again, when the species thus established are compared, certain of them are found to agree with one another, and to differ from all the rest in some one or more peculiarities. They thus form a group, which in one sense is merely a species of higher order, while technically it is termed a "genus." And, by a continuation of the same process, genera are grouped into families, families into orders, and so on. Each of the groups thus named is in the logical sense a genus, of which the next lower groups constitute the species. The characters on which species are based necessarily depend upon the nature of the bodies classified. Thus, mineral species are founded upon purely morphological characters; that is to say, they are defined by peculiarities either of form, color, and the like, or of structure; which last term may be used to include both the physical and the chemical characteristics of a mineral. The distinction between a species and a variety is wholly arbitrary, except so far as it is commonly agreed that individuals which differ from others only as terms of a gradual series of modifications belong to the same species, and are to be considered merely as varieties of that species. It is conceivable that animals and plants should have been known to us only by their remains preserved in museums or in the fossil state. If this had been the case, biological like mineralogical species could have been defined only by morphological characters; that is to say, by the peculiarities of their outward form and inward structure; and, as a matter of fact, this is the state of our knowledge in respect of a large proportion of the existing fauna and flora of the world, and of all extinct animals and plants. A botanist or a conchologist, who sets to work to arrange a newly received collection, sorts out his plants or his shells according to their likenesses and unlikenesses of form and structure, until he has arranged them into groups of individuals which agree in certain constant characters, and differ only by insignificant features, or by such peculiarities as vary in different individuals in such a manner that an insensible gradation can be traced between those forms which have the peculiarity strongly marked and those in which it is absent. Thus far the considerations which guide the biologist in the establishment of species differ in no respect from those which influence the mineralogist. But although naturalists have no more direct knowledge of any but the morphological characters of the great majority of the species of animals and plants than they would have of so many mineral specimens, they are familiar with many animals and plants in the living state, when they exhibit phenomena to which the mineral world presents no parallel; and the study of these phenomena of active life has complicated the conception of species in biology, by adding physiological to morphological considerations. The fact that living beings originate by gener ation from other living beings is one of the circumstances in their history which most completely differentiate them from minerals; and ideas derived from the study of the phenomena of generation enter in various ways into the conception of biological species. For example, it is a generally assumed axiom in biology that whatever proceeds from a living being by way of generation is of the same species as that from which it proceeds, whether the morphological differences between parent and offspring be great or small. The two sexes are often extraordinarily different, and in cases of the so-called "alternation of generations" the successive zoõids may differ very widely; but, inasmuch as the differing forms in these cases proceed from one parentage, no one doubts that they belong to the same species. The breeds of domesticated animals and plants often differ morphologically as widely as admitted species do; but, apart from other considerations, historical evidence that they have the same parentage suffices to cause them to be regarded as of one species. It is not quite clear that the converse of the axiom which has just been referred to would now be admitted, and that living beings which arise from totally distinct parents must be held to be of different species, even though morphologically identical. The well-nigh exploded hypothesis of the multiplicity of centres of origin for species of wide distribution, indeed, implies the belief that groups of individuals which have proceeded from distinctly created parents may nevertheless be of the same species; while the supporters of the no less nearly extinct hypothesis of the independent creation of the faunas and floras of successive formations used to affirm that, although indistinguishable, two animals or plants from separate formations must be of distinct species, because they have been created separately. However, these subtleties have ceased to have any practical importance. In the next place, it is observed that, while individuals of the same morphological species breed freely with one another and give rise to perfectly fertile offspring, the unions of individuals of different morphological species are, as a rule, either infertile or imperfectly fertile. Thus fertility, like parentage, has become a physiological character of species; and though in the case of some domesticated animals, as pigeons, the extreme forms are more different from one another than are many morphological species, yet, apart from the historical evidence of their parentage, they are held to be members of the same species because they are all perfectly fertile one with another, and their offspring are also perfectly fertile. Thirdly, it is a matter of experience that, as a general rule, and taking the whole cycle of forms through which a living being runs into account, offspring and parent are so similar that they belong to one and the same morphological species; and it is further in evidence that many species have endured for extremely long periods species. Varieties are accidental subdivisions of species. Generation being the only means of ascertaining the limits to which varieties may extend, species should be defined, the reunion of individuals descended from one another, or from common parents, or from such as resemble them as closely as they resemble each other; but, although this definition is rigorous, it will be seen that its application to particular individuals may be very different when the necessary experiments have been made." It need hardly be said, however, that in practice Cuvier founded his species upon purely and exclusively morphological characters, just as his predecessors and successors have done. The combination of Cuvier's views on the fixity of species with the discovery of the succession of life on the globe, which was so largely the result of his labors, led his followers into curious difficulties. Developing the fundamental idea of the Discours sur les révolutions de la surface du globe, naturalists were necessarily led to conclude, not only that existing species are the result of creation, but that the creative act which brought them into being was only the last repetition of a series of such acts, by which the often depopulated world has been as frequently repeopled. Lamarck, Cuvier's contemporary and countryman, must be regarded as the chief founder of the reaction against the doctrines which Cuvier advocated; a reaction which, overpowered and disregarded for many years, has acquired such force since and through the publication of Darwin's "Origin of Species," that it has already almost swept opposition away. Lamarck's vast acquaintance with the details of invertebrate zoology rendered him familiar with the great variability of many species, and led him to see that variation is in some way related to change of conditions. The frequent occurrence of transitional forms between apparently distinct species, when large suites of specimens (especially when they are obtained from different parts of a wide geographical area) are examined, tended to bring into strong light the tenuity of the distinction between species and varieties. The facts of embryology, the occurrence of rudimentary organs, and the fundamental unity of structure which obtains in vast groups, such as the vertebrata and arthropoda, further tended to suggest the existence of a genetic connection between the members of these groups; so that Lamarck was induced to renounce the doctrine of the without any notable difference being discern- | limits; and all the beings appertaining to one ible between ancestor and descendant. More- of these forms constitute what is termed a over, in some cases, varieties are found to revert to the characters of the species from which they have proceeded. The conclusion has been drawn that species are physiologically fixed; that is to say, that, however long the process of generation may be continued, the individuals either retain the identical morphological peculiarities of the oldest ancestor, or, if they vary, the varieties remain fertile with one another. Assuming that species have the physiological fixity thus indicated, certain conclusions respecting the origin of species are inevitable. It is clear that no existing species can have arisen by the intercrossing of preexisting species, or by the variation of preexisting species; but that every species must either have existed from all eternity, or have come into existence suddenly in its present form, which is the objective fact denoted by what is termed "creation."-At the dawn of modern biology, a century ago, no scientific evidence respecting the real history of life on the globe was extant, and, for any proof that existed to the contrary, species might have been of eternal duration. But philosophical speculation combined with theological dogma not only to favor the contrary opinion, but to lead the most philosophic naturalist of his day to embody the hypothesis of creation in a definition of species. Totidem numeramus species quot in principio forma sunt create ( We reckon as many species as there were forms created in the beginning"), is the well known formula of Linnæus. In practice, Linnæus regarded species from a purely morphological point of view; in theory, he assumed the ancestral creation and the limited variability of species, though he was disposed to allow more freedom in this direction than most of his successors. On the other hand, he seems to have attached comparatively little weight to the assumed sterility of hybrids, and to have held a sort of modified doctrine of evolution, supposing that existing species may have been produced by the interbreeding of comparatively few primordial forms. It is mainly to the influence of Cuvier's authority that we owe the general acceptance of the views respecting the physiological characters of species which till within the last few years have been almost universally prevalent. In the introduction to the Règne animal (1817), Cuvier writes: "There is no proof that all the differences which now distinguish organized beings are such as may have been produced by circumstances. All that has been advanced upon this subject is hypothetical; experience seems to show, on the contrary, that in the actual state of things varieties are confined within rather narrow limits, and, so far as we can retrace antiquity, we perceive that these limits were the same as at present. We are thus obliged to admit of certain forms which since the origin of things have been perpetuated, without exceeding these fixity of species, and to define a species as "a collection of individuals which resemble each other and produce their like by generation, so long as the surrounding conditions do not alter to such an extent as to cause their habits, characters, and forms to vary." According to this definition, the distinction between species and variety once more becomes conventional. A variety is, in fact, a nascent species; and led to the gradual evolution of species, the hypothesis that they have arisen by such a process of evolution would be the only one which would have any scientific foundation.The great service which has been rendered to science by Mr. Darwin, in the "Origin of Species," is that, in the first place, he has marshalled the ascertained facts of biology in such a manner as to render this conclusion irresistible; and secondly, that he has proved the following proposition: Given the existence of living matter endowed with variability, the interaction of variation with the conditions of existence must tend to give rise to a differentiation of that living matter into forms having such morphological relations as are exhibited by the varieties and species which actually exist in nature. What is needed for the completion of the theory of the origin of species is, first, definite proof that selective breeding is competent to convert permanent races into physiologically distinct species; and secondly, the elucidation of the nature of variability. It is conceivable that both the tendency to vary and the directions in which that tendency takes effect are determined by the molecular constitution of a living body; in which case, the operation of changes of external conditions will be indirect, and, so to speak, permissive. It is conceivable, on the other hand, that the tendency to vary is both originated and directed by the influence of external conditions; or that both variation and the direction which variation takes are partly determined by intrinsic and partly by extrinsic conditions. In this case, surrounding circumstances must be regarded as, to a greater or less extent, the true causes of variation. the notion of the creation of species vanish- | could be offered, as to the causes which have es, inasmuch as every species is the result of the modification of a predecessor. Lamarck's views of the nature of geological change were in harmony with his biological speculations, and wholesale catastrophic revolutions were as completely excluded from the one as from the other. It is impossible to read the Discours sur les révolutions of Cuvier and the Principes of Lamarck without being struck with the superiority of the former in sobriety of thought, precision of statement, and coolness of judgment. But it is no less impossible to consider the present state of biological science without being impressed by the circumstance that it is the conception of Lamarck which has triumphed, and that of Cuvier which has been vanquished. Catastrophic geology has vanished, and is everywhere replaced by the conception of slow and gradual change. With it has disappeared the once prevalent notion that the whole living population of the earth has been swept away and replaced in successive epochs. On the contrary, it is now certain that the changes which have taken place in that population have been effected by the slow and gradual substitution of species for species. Moreover, it is well established that, in some cases, the succession of forms in time is just such as that which should have occurred if the hypothesis of evolution is well founded. The rapid advance of comparative anatomy has diminished or removed the wide intervals which formerly appeared to separate the different divisions of the animal and vegetable kingdoms from one another. Even the hiatus between the vertebrata and the invertebrata is bridged over by recent discovery. The establishment of the cell theory, however much the views originally propounded by Schwann have been modified, leaves no doubt that there is a fundamental similarity in minute structure not only between all animals, but between them and plants; while the discoveries of embryologists have proved that even the most complex forms of living beings do, in the course of their development, run through a series of changes of the same order as those which are postulated by the evolution theory for life in time. Again, the facts of geographical distribution, as now known, are absolutely incompatible with the hypothesis that existing animals and plants have migrated from a common centre, and, by demonstrating the similarity of the existing fauna and flora of any locality to those which inhabited the same area in the immediately precedent epoch, have furnished a strong argument in favor of the modifiability of species. Thus, it is not too much to say that the facts of biology known at the present day are all consistent with and in favor of the view of species entertained by Lamarck, while they are unfavorable to, if not incompatible with, that advocated by Cuvier; and that, even if no suggestion had been offered, or SPECIFIC GRAVITY. See GRAVITY, SPECIFIC. SPECTACLES, contrivances worn to assist sight or to protect the eyes from injury. 1. Spectacles to assist Sight. These may operate in two general ways: first, by correction of some optical defects to which the eyes are liable; and secondly, by compensation for functional insufficiency on the part of certain muscles concerned in the exercise of sight. The eye is a camera, where a system of lenses throws an image upon a screen, represented by the retina. For perfect sharpness of this image, the curves of the lenses must be symmetrical, and the refractive power of the system exactly adjusted to the distance of the retina. In the normal or "emmetropic" eye these conditions obtain, the adjustment being such that when the eye is at rest the rays from distant objects come to an exact focus upon the retina. But every possible deviation from these conditions is found. First, there may be a disproportion between the refractive power of the eye and the distance of the retina. If the refractive power is proportionately too great, the rays from distant objects will come to a focus a certain distance in front of the retina. This constitutes the condition called myopia or near-sightedness, and may arise either from excessive convexity of the lens system of the eye, or from an undue depth of the organ from before backward. The latter origin is by far the more common, and is generally the result of a disease of the tunics of the eye at their back part, whereby being weakened, they bulge out backward. However produced, the correction of myopia is the same. The difficulty being that the refractive power of the eye is too great for the distance of the retina, the obvious remedy is to weaken the former, and this is done by wearing a concave glass. (See OPTICS.) But there are many physiological reasons why full correction of the defect is often improper or useless, which cannot be discussed here. In any but very moderate degrees of myopia glasses should be worn only under competent advice; and in any case great injury may be produced by the use of too strong glasses. The opposite condition to myopia is also very common, that is, where the refractive power of the eye lenses is disproportionately weak, so that the rays from distant objects come to a focus behind the retina, in which case vision of objects both far and near is indistinct. This constitutes the condition known as hypermetropia, and, as in myopia, the deviation from the normal condition may be either in the refractive power or in the depth of the eye. Thus a tolerably common congenital malformation is an undue shallowness of the eyeball. Such an eye is necessarily hypermetropic. A normal eye may also become hypermetropic in old age, and in all cases where the crystalline lens of the eye is wanting, as after removal for cataract (see CATARACT, and EYE), a high degree of hypermetropia necessarily results. The fault being that the refractive power of the eye is disproportionately weak to suit the distance off of the retina, the necessary additional power can be supplied by a convex glass worn before the eye. But in the case of the more common congenital hypermetropia from deficient depth of the eyeball, so many other considerations than the mere optical one affect the matter of correcting the defect by glasses, that perfect neutralization is often unadvisable or unnecessary. For the eye has itself the power of increasing the refraction of its lens within a certain range, to provide for the focalizing upon near objects. (See VISION, Section on accommodation of the eye.) Hence the organ can itself compensate for a certain amount of hypermetropia, and may thus be able to do without glasses, or with weaker ones than those required to neutralize the defect completely. The third optical error remediable by glasses is a certain want of symmetry in the curve of the cornea, where there are two opposite meridians of unequal curvature. This condition is called astigmatism, and is generally a congenital malformation. The consequence of it is that the retinal image, whether of far or near objects, is never sharp. For the correction of this defect a glass is worn having a cylindrical curve equal to the difference in curvature between the two dissimilar meridians, the axis of the cylindrical surface being carefully adjusted so as to be at right angles to the direction of the meridian to be corrected. The nature of the curve, i. e., whether convex or concave, will depend on whether the refractive power of the meridian to be corrected requires to be strengthened or lessened. As it is obvious that this irregularity of corneal curvature may coexist with a general myopia or hypermetropia, compound glasses are often required, having on one face a cylindrical curve to neutralize the astigmatism, and on the other the proper spherical curve required for the other defect.The second general way in which glasses operate to assist sight is, as already said, by compensating for failure of certain muscles concerned in the use of the eyes to fulfil their function. The most common of these troubles is want of power to focalize the eye upon near objects. This faculty resides in a little muscle within the eye, by the action of which the convexity, and thus the refractive power, of the crystalline lens is temporarily increased. But the substance of the crystalline lens steadily grows harder, and thus less and less compressible, so that the same amount of muscular action comes to produce less and less effect. The consequence is that during adult life the focalizing power upon near objects steadily diminishes, and hence the nearest point of distinct vision gets further and further from the eye, until at about the age of 47 it has receded beyond the distance for convenient use of the hands. Reading, writing, sewing, or any manual work requiring sharp vision of small objects at the customary distance, then become impossible without artificial compensation for the failure of focalizing power. This condition, which is natural to all eyes, is called presbyopia or oldsightedness, and the compensation is very simple. The difficulty being an inability on the part of the eye itself to increase temporarily its refractive power, the needed addition is artificially supplied by a convex glass, which is worn of course only when near objects are to be viewed. As the focalizing power keeps on diminishing until in old age it is wholly lost, the strength of the glasses must be steadily increased. As soon as presbyopia begins to show itself, the proper weak glass should be promptly assumed, as only injury to the eyes, or at least useless inconvenience, can result from a fruitless struggle to do without this aid. In all cases the weakest glass with which ordinary type can be clearly and comfortably seen at the usual distance is the proper one to wear. With normal eyes, individuals of the same age take very nearly the same strength of glass, but, for obvious reasons, in myopes the glass will be weaker in proportion to the degree of the optical defect, while in hypermetropes it will be correspondingly stronger. This same inability to focalize upon near objects may also | heavier and far more expensive than glass. The claims for its "preserving the sight " are fanciful, and many of the spectacles sold as pebbles are not such at all. It is always important that the lenses should be of first class, the substance without flaw, and the grinding accurate. A convenient test is to hold the glass some distance from the eye, and then, occur at any age from inherent weakness or paralysis through disease of the muscle concerned, and in such case, as in true presbyopia, a convex glass will be needed for near work. It was probably to compensate for presbyopia by convex glasses that spectacles were first invented. Roger Bacon first pointed out the benefit to old men and "to those that have weak eyes" of viewing letters through a plano-moving it from side to side and to and fro, convex lens. Alessandro di Spina, a monk of Pisa who died in 1313, is generally aceredited with having made public the use of spectacles, which were apparently invented some time between 1280 and 1311.-Another form of muscular insufficiency that can be compensated by optical means is where some of the muscles moving the eyeball in its socket are unduly weak. In such case the holding of the two eyes fixed upon the same point is attended by a feeling of straining or actual pain, and upon prolonged effort the overtaxed muscle may suddenly relax, producing immediately a temporary confusion of sight. Here, if the insufficiency be but slight, the wearing of a weak plain prism, properly adjusted, compensates for the defect; for even while the eyes are allowed to keep the faulty relative position enforced by the muscular weakness, the rays coming from the object desired to be seen can, by means of refraction through a prism, be made to enter both eyes in the same direction, the only condition necessary for binocular single vision. But this mode of compensation will only do in slight degrees of muscular insufficiency; in the higher grades a radical cure by a surgical operation is necessary. The strength and position of the prisms will of course be determined by the degree and seat of the muscular weakness; and if, as is often the case, the affection in question is associated with myopia, hypermetropia, or astigmatism, a compound glass may be needed, where one or both faces of the prism bear the necessary curves to correct the optical defect. The designation of the strength of glasses is nowadays by the fraction expressing the refractive power of the lens in terms of inches, the words "positive" and "negative" or the signs + and - indicating respectively a convex or a concave glass; thus 66 " means a concave lens of 10 in. focal length. Glasses are commonly ground with an equal curve on both faces, but a meniscus for a positive and a concavo-convex for a negative lens may also be used (see OPTICS), in which case the spectacles are called periscopic. The advantage of this form is, that there is less distortion of objects seen through the edges of the lens; but the disadvantages are, that the glasses are heavier than those of the ordinary style, and give more reflection from their back surface. The material for spectacles is commonly glass, but a variety of rock crystal called "Brazilian pebble" is also used. The latter substance is less apt to scratch or to become dimmed by deposit of moisture on being brought from a cold to a warm temperature, but it is note if there be any apparent flickering or distortion of objects seen through it. If there be, the glass is worthless. In style of frames, as is well known, there is great variety. In general the word "spectacles" is now used to designate a frame held in place by bows reaching behind the ears, and "eye glasses" one held in the hand or made to clasp the nose. The spectacle frame is the best where the glass has to be continuously worn, as in myopia, as the lenses can be more accurately centred and made to set perpendicular to the line of sight. For temporary use, as for reading glasses in presbyopia, good eye glasses, selected so as to be well centred to suit the distance of the eyes apart, are convenient and unobjectionable. The material for the frames is various; silver was formerly in general use, but has been superseded by steel and gold. Tortoise shell is light, but easily broken; it is only used in eyeglass frames. The frame, whether spectacle or eye glass, should be selected to suit the individual conformation of face and the purpose for which the glass is wanted, so that the line of sight shall be through the centre of the glass and perpendicular to its surface. Hence glasses for distant vision, as in myopia, should be set high and vertical, while for near work only, as in presbyopia, they should be lower and inclined. A style of spectacles was invented by Franklin for special cases where a different glass is needed for far and near vision respectively, in which the glass is bisected horizontally, the two segments being of the different curvatures required, the upper for the far and the lower for the near. 2. Spectacles for Protection. To shield sensitive eyes from excess of light, colored glasses, either with plane surfaces or of a watch-glass form, are used. The latter give most protection, as they cut off the side light more perfectly. Still better are goggles with wings at the sides. Shades of blue and "London smoke" neutral tint are the best colors. For protection against the glare of snow or white sand, an opaque disk pierced with a narrow horizontal slit is very efficient. A spectacle frame set with wire gauze or plain glass is sometimes worn by workmen as a protection against bits of flying stone or steel. SPECTRUM (Lat., an image), the name given to the image or colored band formed by the decomposition of a beam of light into its elementary colors. Thus, when a beam of sunlight enters a dark room through a narrow slit, passes through a triangular glass prism, and then falls upon a screen, we may observe |