Abstract
THE considerations put forward by Dr. Delbrück and Dr. Timoféeff-Ressovsky are of undoubted importance and were in my mind when my lecture was written. In fact, my suggestions were based on the parallelisms between induced variation and the so-called spontaneous mutations, together with the quantitative relation mentioned in Section 1 of their letter. But I consider that at present the view that ” spontaneous mutation can be explained without an admission of special mutation-inducing agents in the external environment” is an assumption which has yet to be proved. The fact that more spontaneous mutations occur than would be expected to result from natural radiation is not a final objection. For as Prof. Blackett pointed out1 there are certain differences between the nature of the ionisation produced by cosmic rays and by gamma rays, so that it cannot be concluded that the effects of both, when of the same average intensity, are always identical. The differences are connected with the production of bursts and showers. We know that these occur at sea-level2 and in water3 while their frequency increases with altitude4 more rapidly than the cosmic ray ionisation. Thus we are yet scarcely in a position to calculate the mutation rate due to cosmic rays from the X-ray data. Again, some plant species appear morphologically stable over long periods of time, while others are unstable. Apparently X-radiation may set up conditions causing unexpected variation in more than one successive generation of plants5, and I think we may have to distinguish between the manifestation of genetic instability and its original cause.
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Att. 1° Congr. Int. Elet. rad. biol, Venice 1934, 2, 1169 (1935).
For references, see Hoffmann, Internat. Physics Conference London, 1934, 1, 226 ; also Carmichael, H., Proc. Roy. Soc., A (in press).
Weischedel, Phys. Z., 38, 796 (1935). Clay and Clay, Physica, 2, 1042 (1935).
Montgomery and Montgomery, Phys. Rev., 47, 429 (1935).
Prof. Goodspeed at Sixth International Botanical Congress, 1935.
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THOMAS, H. Cosmic Rays and the Origin of Species. Nature 137, 359 (1936). https://doi.org/10.1038/137359a0
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DOI: https://doi.org/10.1038/137359a0
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