The chief evidences in support of nativistic theories are, first, the metamorphopsia after dislocation of retinal elements (p. 119) and, secondly, the position of the line of orientation (p. 131), which indicates united functioning of the two eyes from the first. It has been noted already (p. 120) that the metamorphopsia and other related phenomena prove the exact opposite as soon as the chances to which they are due become stationary. Furthermore, the fact that in long continued use of only one eye the line of orientation comes to coincide with the line of regard (p. 131), proves that the position of this line is not given from the first, but that it has arisen under the influence of the conditions of vision. Still another fact against nativistic and in favor of the genetic theory is the development in the child of the synergy of ocular movements under the influence of external stimuli and the organization of space-perceptions which apparently accompanies it.
Here as in many other respects the development of most animals is different.
In the latter cases the reflex connections of retinal impressions with movements of the eyes and head function perfectly immediately after birth. (v. inf. § 9, 2).
The genetic theory has gained the ascendency over older nativistic and empirical views primarily through the more thorough investigation of the phenomena of binocular vision. Nativism has difficulty with the question why we generally see objects single although they produce images in each of the two eyes. The effort is made to avoid the difficulty by assuming that two identical retinal points are connected with the same optic fibre which divides in the chiasma, and that in this way they represent in the sensorium only a single point. This doctrine of the "identitv of the two retinas" was, however, untenable as soon as the actual conditions of binocular vision in three-dimensions began to be investigated. Especially the invention of the stereoscope thus brought with it a new era for the genetic theory of vision.
[ 1] A process analogous to this elimination of the metamophopsia is sometimes observed in binocular vision when the disturbances arising from squinting are gradually overcome. When the squinting begins, the two lines of regard no longer meet in the field of vision, so that double images of objects arise. These may gradually disappear, however, if the condition of the eyes remains perfectly stationary; a new set of relations is developed for the retinal elements of the squinting eye.
[ 2] In this connection, we have the fact that the blind spot does not appear as a break in the field of vision, without sensational contents, but as a continuation of the general brightness and color of the whole field; for example, as white when we are looking at a white surface, as black when we look at a black one. This filling out of the blind spot is possible only through reproduced sensations, and is to be considered as one of the phenomena of association to be discussed later (§ 16).
[ 3] The habit of seeing with two eyes results in exceptions to this rule. Often when one eye is closed, the line of orientation remains the same as in binocular vision and does not coincide with the line of regard. In such cases the closed eye usually makes the corresponding movements of convergence upon a common fixation-point with the open eye.