RETROSPECT AND PROSPECT 



originally Palearctic animals, have been inadvertently carried to all parts 

 of the w^orld on cargo ships. They live in direct competition with man, 

 invading his buildings for shelter, and robbing his larder for food. On the 

 whole, they have been so successful that they must be very much more 

 numerous than they were before the rise of civilization. The crow similarly 

 has profited by farm lands, and small carnivores, such as foxes and weasels, 

 have not only learned to live in close proximity to man, but they have 

 learned to attack his small domestic animals, such as chickens and rabbits, 

 and still escape his wrath sufficiently well to maintain themselves in com- 

 petition. The success with which many weeds have invaded civilized habi- 

 tats is too familiar to require discussion. 



Radiation and Evolution. Another activity of man has great potential 

 importance for evolution. This is the induction of mutations by radiation, 

 including atomic radiation. Since the discovery by Muller that mutations 

 could be induced by X-radiation, the possibilities have been intensively 

 explored. Any high-energy radiation will cause mutations in numbers 

 directly proportional to the total dosage of radiation. Although the possi- 

 bility of using radiation to induce useful mutations has been extensively 

 investigated, especially in crop plants, almost all of the experimentally 

 produced mutations have been deleterious. A brilliant exception is pro- 

 vided by the work of Demerec on PenicilUum notatuni, the mold which 

 produces penicillin. Previous to Demerec's experiments, the best available 

 cultures of PeniciUium produced about 70,000 units of the drug for every 

 pint of culture. He X-rayed large numbers of the organisms, and carefully 

 tested penicillin production of pure cultures derived from X-rayed mold. 

 Most of the X-rayed stock was of no special value, but one highly produc- 

 tive strain, yielding 280,000 units of penicillin per pint of culture, was 

 obtained. It is from this stock that most commercial penicillin is now pro- 

 duced. While the possibility that other valuable mutations may be pro- 

 duced by radiation cannot be entirely ruled out, still it is apparent that 

 radiation is, on the whole, a genetic and evolutionary hazard. In a society 

 in which radiation is playing an increasing role in medicine, experimental 

 science, and even industry, it is impossible to estimate what the eflFects of 

 inadvertent radiation of the gonads may be, both for man and for his asso- 

 ciated plants and animals. 



Such considerations led Muller to publish on the cover of the Septem- 

 ber, 1947, issue of the Journal of Heredity the warning that ". . . like most 

 species, we are already encumbered by countless undesirable mutations, 

 from which no individual is immune. In this situation we can, however, 

 draw the practical lesson, from the fact of the great majority of mutations 

 being undesirable, that their further random production in ourselves 

 should so far as possible be rigorously avoided. As we can infer with cer- 

 tainty from experiments on lower organisms that all high-energy radiation 

 must produce mutations in man, it becomes an obligation for radiologists 

 —though one far too little observed as yet in most countries— to insist that 

 the simple precautions are taken which are necessary for shielding the 

 gonads, whenever people are exposed to such radiation, either in industry 

 or in medical practice. And with the coming increasing use of atomic 



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