dation of distinctness of vision is its representative, and therefore its sign, in the field of view.

Now, as we go down the vertebrate scale, the central spot is found only in the higher monkeys. After a total absence in all other mammals and all birds, it is said to reappear in some lizards, especially the chameleon. But whether in these the organization of this spot is similar to that in man-whether it is really a central spot in the same sense, and has the same significance in vision or not-may be still a question. It seems fair to conclude, therefore, that the graduation of distinctness toward the point of sight, and the limitation of the greatest distinctness to that point, which we find in man, do not exist, at least to the same degree, in most of the lower animals.

The importance of a central spot in the highest animals, and especially in man, is very evident. The limitation of the greatest distinctness to the point of sight is absolutely necessary to the concentration and limita tion of the most thoughtful attention to that point. If all portions of the retina were similarly organized, and therefore all points in the field of view equally distinct, it would be impossible to fix the attention steadily and thoughtfully on any one point to the exclusion of others. We might see equally well, and over a wider area; but we could not look attentively at anything; we could not observe thoughtfully. But in the lower animals, especially those, as the ruminants, which are preyed upon by others, it is far more important to see well in every direction, than to fix attention exclusively on one point; therefore the advantages of exquisite microscopic distinctness of the center of the field is sacrificed for the much greater advantages of moderate distinctness over a very wide field. The most

important thing for them is a very wide field and the equal distribution of attention over every part. Hence their eyes are prominent, set wide apart on the margins of a broad front, and destitute of central spot; so that they sweep the whole horizon, and see all parts with nearly equal distinctness.

It may be said that the sight of these animals is equal or even superior to that of man, and therefore the organization of their retina is probably as fine as that of our central spot. I answer that there are two things to be considered in this connection. The one is sensitiveness to light, and therefore perception of the presence of objects; the other is distinctness of the perception of form. The one gives us notice of the existence of objects, the other gives us distinct knowledge concerning these objects. It is this latter which depends on the fineness of organization of the bacillary layer. Other portions of the human retina are even more sensitive to light than the central spot, as is shown by the well-known fact that we see a faint star by looking a little way from it, when we can not see it by looking directly at it. But distinctness of form is perceived only by the central spot. It seems probable, therefore, that animals destitute of a central spot, although they may have a more delicate perception of the existence of objects in the field of view than we, yet do not see the form of objects regarded as distinctly as we do. For this reason they are more apt to mistake the nature of objects, and therefore more easily frightened by trifling

causes.

Again, it is well to observe that the chameleon, in which the central spot seems to reappear, is an animal whose habits and mode of taking its food require the most fixed and undivided attention.

The close connection of the central spot with binocular vision is also quite evident. The central spot, more than all other portions of the retina, is endowed with the properties of corresponding points; and the somewhat complex binocular judgments expressed by the term "stereoscopic perspective" are accurate and reliable only at and in the vicinity of the point of sight. This fact constitutes the great difficulty in the way of the experimental determination of the horopter, as already explained (page 197). It is therefore, to say the least, doubtful if animals whose eyes want the central spots are able to judge as accurately of the relative distance and the solid forms of near objects as we do.

The following, then, are the general changes in the vertebrate eye as we go up the scale: 1. A gradual change of the position of the eyes from the sides to the front of the head, and a consequent change of the angle of inclination of the optic axes from 180° to parallelism; 2. A regularly increasing graduation in the fineness of the bacillary layer of the retina, and therefore in the accuracy of the perception of form, from the anterior margins toward the central parts, so as finally to form in monkeys and in man a specially organized central spot; 3. A gradually increasing power of converging the optic axes on a single near point, so that the images of that point may fall on the central spots of both eyes; 4. The gradual evolution of the properties of corresponding points, and therefore of all the distinctive phenomena of binocular vision.

These changes seem all intimately connected with each other and with the development of the higher faculties of the mind.