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.
Fig. 9. Captorhinus. Diagram, showing some hypothetical lines of stress. Approx. × 1.