The second course has been the attempt to explain the appearance of temporal openings in whatever line in which they occurred by the action of the same constant selective force. According to the reasoning of this theory, temporal fenestration in all groups was due to the need to decrease the total weight of the skull, and selection in all those groups where temporal fenestration occurs was to further that end. Both of these routes of inquiry are inadequate. If modern views of selection are applied to the problem of explaining the appearance of temporal fenestrae, the possibility cannot be ignored that: 1. Selective pressures causing the inception of temporal fenestrae differed from those causing the continued expansion of the fenestrae. 2. The selective pressures both for the inception and continued expansion of the fenestrae differed from group to group. 3. Selection perhaps involved multiple pressures operating concurrently. 4. Because of different genotypes the potential of the temporal region to respond to selective demands varied from group to group. [Illustration: FIG. 9. _Captorhinus._ Diagram, showing some hypothetical lines of stress. Approx. × 1.] [Illustration: FIG. 10. _Captorhinus._ Diagram, showing areas of internal thickening. Approx. × 1.] [Illustration: FIG. 11. _Captorhinus._ Diagram, showing orientation of sculpture. Approx. × 1.] Secondly, the vectors of mechanical force associated with the temporal region are complex (Fig. 9). Presumably it was toward a more efficient mechanism to withstand these that selection on the cheek region was operating. The simpler and more readily analyzed of these forces are: 1. The force exerted by the weight of the skull anterior to the cheek and the distribution of that weight depending upon, for example, the length of the snout in relation to its width, and the density of the bone. 2. The weight of the jaw pulling down on the suspensorium when the jaw is at rest and the compression against the suspensorium when the jaw is adducted; the distribution of these stresses depending upon the length and breadth of the snout, the rigidity of the anterior symphysis, and the extent of the quadrate-articular joint. 3. The magnitude and extent of the vectors of force transmitted through the occiput from the articulation with the vertebral column and from the pull of the axial musculature. 4. The downward pull on the skull-roof by the adductor muscles of the mandible. 5. The lateral push exerted against the cheek by the expansion of the mandibular adductors during contraction. 6. The necessity to compensate for the weakness in the skull caused by the orbits, particularly in those kinds of primitive tetrapods in which the orbits are large. The distribution of these stresses is further complicated and modified by such factors as: