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Show 186 DR. G. LINDSAY JOHNSON ON THE [Jan. 19, as we wend our way from the Lemurs through the Galagos clown to the Aye-Aye. The eyes of all the Primates below M a n are smaller than our own, but this is not so in proportion to the size of the body. I have found, for instance, that the transverse diameter of the globe of the eye of the half-grown Gorilla which lately died in the Society's Gardens measured 20-7 mm., which is the size of the eye of a child between the age of 9 and 11. Of course the eye of a small Marmoset is very much smaller, being in proportion to the size of the animal. There are other distinctive differences between Man, the Monkeys, and Marmosets, in other words between the Anthropoidae on the one hand and the Lemuroidae on the other. The pupil of M a n and the Anthropoidae is always circular, whilst w e find a vertically oval pupil in all the Lemuroida?. In addition to this I find from repeated observations that all the Anthropoidae or true Monkeys are able to accommodate their eyes for near objects by converging both eyes on to a single point, and in so doing the pupil contracts as in M a n , though to a less degree. The Lemuroidae have not this power of convergence ; and although I have noticed the power of convergence in all the Monkeys, 1 find it is only a transition stage-that is to say, they employ it with hesitation and difficulty, much in the same way as an infant uses its legs when learning to walk, since they cannot converge for more than one or two seconds at most. If you hold a small bright-coloured object near the nose of a Monkey, you will observe the eyes converge immediately in a horizontal plane, and the pupils contract slightly, but the next moment the eyes return to parallel vision, though not necessarily in a horizontal plane, being often accompanied by a slight elevation upwards. WTe thus find that w e must draw a broad distinction between M a n and Monkeys as a group and the Lemuroidae. Man and all the Monkeys and Marmosets without exception possess a macula, a circular pupil, and converge when accommodating for near objects. These characteristics are necessary for binocular vision. The Lemuroidae have not got binocular vision and therefore we find all these peculiarities absent. Going more into detail we find that every family has some characteristic peculiarity. Thus the eyes of the Gorilla, Chimpanzee, and the Orang closely resemble that of the negro, except that around the disc the whitish fine streaks are more marked. Were they as strongly marked in M a n they would be attributed to a congenital defect known as opaque nerve-fibres, although the defective human eye shows these opaque nerve-fibres wholly opaque, whilst in these Apes they are more or less translucent. These translucent nerve-fibres radiating from the disc become somewhat more marked as we descend the scale. In the Gibbon we find an extreme prominence of the choroidal vessels. In the next family, the Cercopithecidae, we notice in some genera an approximation to the Simiidae, notably in the Black Ape, |
