236 SPECIES the notion of the creation of species vanish- es, inasmuch as every species is the result of the modification of a predecessor. Lamarck's viuws <>f the nature of geological change were in harmony with his hiological speculations, and wholesale catastrophic revolutions were as completely excluded from the one as from the other. It is impossible to read the Dis- cours tur les revolution* of Cuvier and the Principe* of Lamarck without being struck with the superiority of the former in sobriety of thought, precision of statement, and cool- ness of judgment. But it is no less impossi- ble to consider the present state of biological science without being impressed by the cir- cumstance that it is the conception of La- marck which has triumphed, and that of Cu- vier which has been vanquished. Catastrophic geology has vanished, and is everywhere re- placed by the conception of slow and gradual change. With it has disappeared the once prevalent notion that the whole living popu- lation of the earth has been swept away and replaced in successive epochs. On the con- trary, 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 evo- lution is well founded. The rapid advance of comparative anatomy has diminished or re- moved the wide intervals which formerly ap- peared to separate the different divisions of the animal and vegetable kingdoms from one another. Even the hiatus between the verte- brata 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 de- velopment, run through a series of changes of the same order as those which are postu- lated 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 SPECTACLES could be offered, as to the causes which have 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 mar- shalled the ascertained facts of biology in such a manner as to render this conclusion irresis- tible ; 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 dif- ferentiation 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 va- riability. It is conceivable that both the ten- dency 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 ex- ternal 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 great- er or less extent, the true causes of variation. SPECIFIC GRAVITY. See GRAVITY, SPECIFIC. SPECTACLES, contrivances worn to assist sight or to protect the eyes from injury. 1. Spec- tacles to assist Sight. These may operate iu 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 exact- ly adjusted to the distance of the retina. In the normal or "emmetropic" eye these con- ditions obtain, the adjustment being such that when the eye is at rest the rays from distant objects come to an exact focus upon the ret- ina. 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
