Lower Animals.

Fig. 61.—Slow loris—right foot.

At the other end of the scale the scanty ridges of a hedgehog’s or squirrel’s foot would be negligible in preventing slipping, however useful they would be, as I hold, as early organs of touch. Between these extremes the slow loris affords a valuable example to study, with the help of Fig. 61. The foot, as more concerned with preven­tion of slipping than the hand, is chosen for observa­tion, but with little exception the hand agrees closely with it. On the tips of four digits, D 1, 3, 4 and 5, omitting D 2 for the moment, the ridges are arranged nearly in a longitudinal direction, and would on that account have little or no effect in preventing slipping of the foot. If this be disputed one can but reply that if the need of preventing slipping in this tiny area were to call forth selective value this is not the arrangement of the ridges that best serves the purpose. It may be remarked here that the pulps of lemurs, the marmoset and squirrel monkey all show this indifferent mode of grouping of ridges. The aborted D 2 of the loris, with its hooked nail overhanging the circular pattern of ridges, is obviously quite unadapted for any non-slipping effect of its skin, as a glance at the figure shows. On the remaining segments of the digits the ridges in the main slope from each side of each digit in the distal direction and fail here also to obtain the best, or transverse direction for preventing slipping in locomo­tion. The corresponding surface of D 1 is not different from its pulp as to direction of ridges, and it is here to be noted and admitted that when this muscular great toe is tightly applied to a branch, which from its shape it must cross at a right angle, the non-slipping effect of the longitudinal ridges would be very effective. One must then notice that over the middle of the sole of this foot the ridges have again changed their direction and lie in a transverse direction. Between this and the basis of the digits are three fleshy pads and an intervening area of longitudinal ridges.

The first question that arises in the attempt to analyse so complex a grouping on a strange member like the foot of a loris is this—what is the primary function subserved by the ridges and their mode of arrangement, and what may be their secondary uses? In the book referred to I have maintained throughout, in opposi­tion to Mrs. Wilder Harris and others such as Dr. Hepburn, that the sense of touch is the primary, and preven­tion of slipping the secondary adapta­tion secured by the ridges. If this be true (and I know it is sub judice) there is a very clear reason why the ridges should be longitudinal on the tips of the digits on account of the better discrimina­tion of small objects secured by this arrangement, though it does not well assist the loris to avoid slipping. On D 1, as mentioned, the non-slipping effect is secured by its ridges, and this digit is necessarily less employed for discrimina­tion than support. On the other hand the sloping arrangement on the rest of the segments of D 3, 4, 5 is decidedly less effective in preventing slipping than a transverse arrangement would have been. I think I am justified in saying that too much has been made of this secondary effect of the ridges in the preven­tion of slipping. I know that the string wound round the handle of a cricket bat is very effective for its purpose, but one can also understand that a casual strand wound here and there on the handle as the ridges are on a hedgehog’s and squirrel’s hand and foot would be of little use for the purpose.

On the other hand if the view may be entertained that on the palm and sole of hedgehog, squirrel, loris and man, we have written in rows of papillary ridges and their modes of arrangement a register of long-continued flexion of hand and foot in flexion and correlated actions, we find the facts of these and numerous other Primates agree in a remarkable manner with the hypothesis; whereas the exclusive non-slipping rival has many awkward facts to explain, or disregard.

Further as one has always to bear in mind the Mendelian analysis it should be observed that the extreme variability, within certain limits, of the arrangements of papillary ridges throughout the Primates renders the hypothesis of unit-characters segregated, according to Mendelian laws, wholly inapplicable to the manner of their arrangement even though perhaps not so to the existence of papillary ridges.

Fig. 62.
Hedgehog—right foot.

Fig. 62A.
Hedgehog—right hand.

Fig. 63.
Common squirrel—left foot.

It may be bluntly asserted that the ridges are arranged as we find them because, hands and feet being used as they are, the ridges “can do no other,” and that there’s an end of it, and that we cannot derive any help as to the origin of specific difference from such a trifle, the next item on the agenda should be called for. As a piece of dialectics that would be effective, but if taken literally it only goes to prove my simple conten­tion.

It will be enough to mention the hand alone of the remaining series with a note as to each animal.

Fig. 64 gives the hand of a chimpanzee with ridges on the pulps resembling those of all the apes, monkeys and lemurs, arched groups on the digits and longitudinal ones on the centre of the palm, both of these last two being exactly what would be found arising from the actions of climbing branches and discriminating globular objects in the palm.

Fig. 64.—Chimpanzee—right hand.

Fig. 66.—Orang—right hand.

Fig. 65 is that of a gorilla and its general features resemble closely those of the chimpanzee and of Fig. 66 which is that of an orang.

Fig. 65.—Gorilla—left hand.

Fig. 67.—Gibbon—left hand.

Fig. 67 of a Hainan gibbon is very different on the palm from the other three apes for its ridges are nearly all longitudinal or slightly oblique, precisely as one would find this part if the palm were used very little for grasping boughs and much for discriminating globular objects procured for its repasts. The wonderful long digits of the gibbon form its main organ for supporting itself on branches and swinging its body rapidly from branch to branch, and the arched or nearly transverse ridges on the digits are placed just as the endless use of them for this purpose would be likely to follow from it. This example is a very clear one for showing, if it exist, the effect of use and habit on the disposi­tion of the ridges.

Fig. 68.—Left foot of ring-tailed lemur.

Fig. 69.—Brown sapajou, right hand.

Fig. 68 shows the arrangement of papillary ridges in a lemur and 69 that of a brown sapajou.

Fig. 70.—Left hand of chacma baboon.

Fig. 70 of the Chacma baboon, playfully called by the Boers Adonis, is a very active and wary animal which lives on the rough rocky slopes of the Cape. It is very much of a pedestrian and the response of its mode of life and use of its forefoot is shown in five great pads of muscle and efficient whorls of ridges for touch, those on the digits being very nearly all transverse in accordance with simple flexion of these joints. This again is what one would expect if my hypothesis be sound. The purely non-slipping mechanism supposed by the rival view is not here well supported by the facts.

Neither the arrangements of ridges (Fig. [61]), in loris, nor the hedgehog (Fig. [62]), nor the squirrel (Fig. [63]), need further reference, but they are all, I think, very consistent with the prolonged effects of use and habit.