ExPLANAioK-i Note to Figure 459 



The several horizons of cerebral development indicated by the figures on the opposite page require some 

 further comment with reference to their evohitional significance. 



I. The Lemuroid Horizon is here considered the basic primate level. It contains many brain features, still 

 in the crude, which become dominant in later development. It also illustrates such hesitations and indecisions 

 as might be inherent in a momentous transitional stage. Faintly, at least, it shows the first feeble impressions 

 imparted to the brain by adaptation to arboreal life and indicates the general lines of cerebral advance 

 consequent upon tree-living habits. (Tarsius and Lemur.) 



II. The Simian Horizon illustrates the dehnite crystallization in cerebral architecture of those structural 

 features which result from the adaptations to arboreal habitat. It clearly reflects the attainment of quadru- 

 manous specialization, at the same time disclosing the ellects ol certain restrictive influences, such as prono- 

 grade locomotion and the as yet partial lilxTation of the forelimbs from locomotory function. (Old and New 

 World Monkeys.) 



III. The Proanthropoid Horizon is epochal in its ellects. A new ty|)e ol adaptation has reorganized the 

 proportions and postures of the entire body. Brachiation is now substituted for a pronograde locomotion in 

 which latter the upper surfaces of the branches are grasped by the handlike fore and hind extremities. Swing- 

 ing from the branches, as do the Gibbons, has produced a marked elongation of the hands, arms and trunk. 

 It has further caused the body to assume a more [jcrpendicular position in passing through the trees and a 

 real erect posture upon the ground. The Gibbon can in fact stand, walk and run upright. It has, in addition, 

 lost its tail, due undoubtedly to its well-established habit of sitting upright in true anthropoid fashion. The 

 animal is, nevertheless, much inferior in its cerebral organization to the higher anthropoid apes and is hence 

 regarded as a representative of that proanthropoid stage whose arboreal readjustments laid tlie foundations 

 for the development of the great a|)es and man. 



IV. The Anthropoid Horizon shows a further specialization in the direction of the ultimate erect posture of 

 man and the final freeing of the hands for purposes other than tho.se of locomotion. Arborealization still 

 exerts such a potent influence upon all of the three great apes, that their advances in the direction of anthro- 

 poid specialization are definitely restrained by this factor. While all of the three great apes, Orang-Outang, 

 Chimpanzee and Gorilla, habitually walk in .'i modified pronograde manner, using the knuckles of the 

 extended hand for support in walking and running, these animals are capable ol standing, walking and 

 running upright. The erect posture, under these circumstances, has little of the perlection attained by man. 

 It is both ineffectual and ungainly. Running and walking are done with a shuffling, waddling gait, with a 

 tendency to come down upon all fours whenever speed is necessary. The great weight of the anthropoid .apes 

 has enforced upon them .m arborco-tcrrestri.il niixlc i>f life. This is more particularly true of the Gorilla 

 which, although it does not resort to anything api>roaching brachiation in its arboreal locomotion, does 

 employ its massive arms for reaching up to the branches, thus drawing itself upward from the ground. All 

 three ol the great apes appear to Ik- olfshoots fniin the |)r(>.inthro|)oid stem; while still another <jff shoot gave 

 rise to the races of men. 



V. The Human Horizon demonstrates certain anthropomorphous conclusions which were first suggested 

 in the proanthropoid stage. Effectual erect posture is at length attained, the hands liberated from purposes 

 of locomotion, speech acquired, and the frontal area of the brain greatly expanded. The primate tendency 

 so obviously introduced on the Lemuroid Horizon has, under the influence of arborealization, progressed 

 to a proanthropoid stage whose essential contribution is the inception of the erect posture. Two major lines 

 of anthropoid derivation induced the adaptations which led respectively on the one hand to the great apes 

 and on the other to man. Whetlur tin- .inthropoid stem was a separate offshoot from the proanthropoid level, 

 or whether several such olfshoots developed, is still a question. The morphological constituents of the brain 

 strongly suggest that the departure of man from the proanthropoid level was at first by a stem in common 

 with the Chimpanzee and Gorilla, with subsequent bifurcation leading to the wide divergence between the 

 human and his neighboring coordinates among the great apes. 



[988] 



