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Mutation Theory

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MUTATION THEORY (Lat. from inutarr, to change, for nioriture, frequentative of inorere, to 111OVV, Skt. ?air. to push). The hypothesis that evolution (q.v.) takes place by means of sudden changes rather than slow: and almost imperceptible transformations. This idea. though 1101 11eW, has been called into great prominence by the publica tion in Pail of Die .11uhilionstheorie by Prof, Bug, de professor of botany at Amster dam. Darwin himself, espeeially in his earlier works, recognized that 'single N':1 l'int ions' III' sport. are to be reekoned with as well as natural selection. Through the influence of 1Vallace and others, natural selection had 11 come in he the I-til ing theory. and it is only in revent years that the signifieance of sports in evolution Las had touch place. 1 lowever, as lung ago as 1 R6 4 I:rdliker made an appeal for het (which is identical with time mutation of De Vries) as a faqir in the evolution of species. Professor Scott of Princeton has used the word mutation for gradual modifications. Still other writers have referred to the sudden origin of species as saltatory evolution. But De Vries is the first author who has performed experiment and worked out a theory to tit the facts which they have yielded.

Before taking up the experiments it may be said that mutation involves no necessary aban donment of natural selection. except in so far as it inay have been held to account for the actual origination of new eharacters. Darwin recognized that natural selection improves hut cannot originate anything now mutation, on the other hand, is a means for the Ilevelopnwnt of new characters, and even a new asseinblag,e characters, i.e. a new species. It should be stated. however, that DeVries gives but little place to natural selection. even as a means of improving someth ing already present. II is experiments show that there is a definite and rather narrow limit to individual variation. and the full ad vantage of artificial selection along any given line can usually be obtained within a few genera tions, as in the parsnip or carrot. lie claims also that natural selection never fixes a character, but that reversion to the original may occur after many generations. :Mutation, on the other hand, is believed to bring into existence some thing wholly new-, without any transitions or con necting links. The new- form. i.e. the mutant, remains 13xed from the outset, and if it is fit. it will remain as a new speeics.

In IS;::01 De Vries observed a colony of evening primrose (Wnothera Lanwrekiana I in which were two forms differing strikingly from the com mon type. Feeling that these aberrant forms doubtless came from the same parentage as the more common forms, he made artificial eultures to see if further aberrant forms would develop.

Ile found such to be the ease to an astonishing degree. Out of 50.000 seedlings of (Enothera Lanoirekintor in the years of study, SOO. or about 1.5 per emit., were mutants, while 98.5 per cent. came true to seed. More than one fourth of the S00 mutant, were of one type. which De Vries named (Enothera iota, i.e. this form appeared in cultures more than '200 times. A form which he called (Enolhcrel giyns appeared but once. \Vhen cultures are made from these new forms they are found to come true to seed except for occasional mutations; nor was any tendiney to reversion observed. To these DOW forms, as to the two aberrant types observed in 85-6, lie Vries has given names which imply that these forms are actually new species. They have, lie says, all the characteristics of species, differing not in one. but in several characters. and remaining eonstant in all vultures. Ile shows that they are as much entitled to be eonsidered separate species as are the various members of the natural group to which the parent Wan/here, 11/Hard-Mau belongs. Indeed. it is possible to identify most of the new specie:: even while yet seedlings. There are no transitions between parent and offspring. or between one new species and another. There is no slow and gradual fixa tion of characters by natural selection, but the new is eomplete from the outset. Most of these new forms would doubtless disappear in a natural state, for they would he 0bli7ed to en gage in a keen struggle with species already pres ent, or would gradually suffer the loss of their specific characters through hybridization. How ever, some new species, especially if stronger than the parent form, might survive and extend their area"; the two aberrant types first observed in a field in ISSG are still to be seen there and in greater numbers. Thus it may be said that De Vries has seen the birth of new species, a phenom enon which all evolutionists have wished to see, is of interest to observe that independently and almost simultaneously with De Vries, Kor sehinsky has brought together a vast mass of data under the title "Heterogenesis and Evolu tion." From the records of gardeners and horti culturists he has concluded that most cultivated 'varieties' of plants have arisen suddenly as sports, i.e. through heterogenesis or mutation.

ButuoutArnr. De Vries, Die .11 ntalionsac oric, vol. i. (Leipzig, 1901); Korsehinsky. Flora (1901) ; White, Ncicnee, vol. ) ; Millet 04 of Torrey Botanical Club (August. 19(l2) ; Bio/ogisches Centralblatt (1901-02) ; De Vries, lleruc Genjrale Botanique, vol. xiii. (1901) ; Science, Vol. XV. ( 1902 ) .