1. Two replicate experiments were made, in which treatment by ether vapour was given to the eggs of Drosophila melanogaster 2 1/2--3 1/2 hours after laying, and selection practised either for the tendency to produce bithorax-like phenocopies ('upward selection'), or against it ('downward selection'). The two foundation stocks were both Oregon-K wild types, but had been bred separately for some considerable time. 2. Selection was effective in both directions, but its progress was not followed in detail owing to the death of many extreme phenocopies in the puparia. 3. In the 8th and 9th generations of the Up selected line of Experiment II, flies which showed a slight enlargement of the halteres appeared among the untreated individuals. These gave rise to a series of stocks, which were found to contain a dominant allele of Bxl with recessive lethal effect. 4. In the 29th generation of the Up selected line of Experiment I, similar flies were again found among untreated individuals. These gave rise to a stock (He 17), which contained a factor which was indistinguishable from that mentioned in para. 3. It might, indeed, have arisen through contamination by flies carrying that gene; but the numbers involved in the experiment (c. 150,000) make it conceivable that both occurrences of the gene were due to independent chance mutations. 5. In the 29th generation of the Up selected line of Experiment I there also occurred, among untreated individuals, some showing the metathorax partially converted into a mesothorax. These gave rise to a stock He*, in which the bithorax phenotype appears in high frequency and extreme grade. The genetic basis of the character is partly a recessive gene which causes females to produce eggs developing into bithorax phenotypes (a maternal effect), and partly a number of minor genes on both the second and third chromosomes. 6. Selection for sensitivity to one type of environmental stimulus does not necessarily produce susceptibility to stimuli of a different type. 7. The fact that such a bizarre phenotype as bithorax can be assimilated, with high grade expression, in less than 30 generations, suggests that the genetic assimilation mechanism is a very powerful one, which could have far-reaching effects during evolution.
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