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Again we emphasize that the interpretation yields only the fact that the lower bound for the mean yield strength is lower slightly above 300 km than below 300 km; similar comparisons apply to the depths slightly above and below 550 km. Strengths are not compared at widely separated depths. No statistical analysis of these results has as yet been made. The statistical properties of the curve may modify the interpretation.

and Summary.-In an attempt to elucidate the genetic architecture of two behavioral traits, populations of Drosophila pseudoobscura were selected for positive and for negative phototaxis and geotaxis.The selected populations diverged rapidly in their behavior ( Figs. 1 and 2). The selection was relaxed after 20 generations in the phototactic, and after 30 generations in the geotactic populations. The relaxation resulted in convergence almost as rapid as was the divergence under selection. The average phototactic and geotactic neutrality of natural populations is an adaptive trait protected by genetic homeostasis. This does not preclude rapid responses to artificial, and presumably to natural, selection.Hirsch and his students1-4 have constructed an ingenious apparatus-classification mazes-in which the reactions of Drosophila flies to gravity and light may be studied quantitatively. Using these mazes, they have selected populations of Drosophila melanogaster genetically geopositive or geonegative, and photopositive or photonegative. Our experiments were made on D. pseudoobscura with mazes modeled on those of Hirsch and of Hadler.4 We also have obtained geopositive, geonegative, photopositive, and photonegative populations.5-8Benzer,9 using a countercurrent distribution technique, selected mutants in D. melanogaster which reacted differently to light. If we understand his technique correctly, he may be dealing with kinds of behavior not identical with those studied in Hadler's and in our experiments-an escape reaction rather than a choice by the flies of light or of dark passages. However that may be, the behavioral variations involved clearly have genetic components.When tested in the Hirsch-Hadler classification mazes, most strains of D. pseudoobscura are, on the average, neutral to light and to gravity.10 Strains of a related species, D. persimilis, are on the average photopositive, but neutral to gravity.10 The behavior of all populations with which experiments were made proved to be easily modifiable, in the positive as well as in the negative directions, by artificial selection. The average neutrality of wild flies is evidently the result of a balance between the genetically positive variants and the genetically negative ones. Though we do not know why this should be so, populations of D. pseudoobscura in nature must be maintained by natural selection in a state of average photo-and geoneutrality. The experiments reported in this article were designed to throw some light on the genetic architecture of the populations responsible for this situation.

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