Aber bis jetzt leider sind die Versuche nicht gelungen, weder mir noch irgend einem anderen.
CHAPTER X
Effects Of Changed Conditions Continued
The Causes of Genetic Variation
In the last chapter we examined some of the evidence offered in support of the belief that adaptation in highly organised forms is a consequence of the inheritance of adaptative changes induced by the influence of external conditions. The state of knowledge of this whole subject is, as I have said, most unsatisfactory, chiefly for the reason that in none of the cases which are alleged to show a positive result have two observers been over the same ground, or as yet confirmed each other. In the wider consideration respecting the causes of variation at large we find ourselves still in the same difficulty. The study has thus far proved sadly unfruitful. In spite of the considerable efforts lately made by many observers to induce genetic variation in highly organised plants or animals, and though successes have occasionally been announced, I do not know a single case which has been established and confirmed in such a way that we could with confidence expect to witness the alleged phenomena if we were to repeat the experiment. Abundant illustrations are available in which individuals exposed to novel conditions manifest considerable changes in characters or properties, but as yet there is no certain means of determining that germ-cells of a new type shall be formed.
Of the direct effect of conditions the lower organisms, especially bacteria, offer the best examples, the alterations of virulence which can be produced in so many distinct ways being the most striking and familiar. That attenuation of virulence can be produced by high temperatures or by exposure to chemical agents, and that this diminution in virulence may remain permanent is, from our point of view, not surprising; but the fact that in many cases the full virulence can by suitable cultivation be restored is difficult to understand. Similar variations have been observed in power of pigment production and other properties.
These phenomena naturally raise the question whether any cases of apparent loss of factors in higher forms may be comparable.
The subject of variations in the lower organisms and their dependence on conditions is a highly special one, and I have no knowledge which can justify me in offering any discussion of them, but I understand that hitherto little beyond empirical recognition of the phenomena has been attempted. A useful summary of observations made by many investigators was lately published by Hans Pringsheim,[1] who enumerates the different agencies which have been observed to produce modifications, and the various ways in which these changes are manifested. One of the most comprehensive studies of the subject from the genetic point of view is that made by F. Wolf.[2] In his extensive cultivations of Bacillus prodigiosus, Staphylococcus pyogenes and Myxococcus he succeeded in producing many strains with modified properties. In most of these the modifications arose in consequence of the application of high or low temperatures or of the addition of various chemical substances to the culture-media. Some of the variations, which are for the most part in the powers of pigment-formation, persisted when the strains were returned to normal conditions, and others did not. In reference especially to the variations witnessed in the Cocci the reader should consult the critical account of variation in that group published by the Winslows,[3] where much information on the subject is to be found. The authors attempted to determine the systematic relationships of the several forms, as far as possible, by the application of statistical methods. The result is interesting as showing that the problem of species in its main features is presented by these organisms in a form identical with that which we know so well in the higher animals and plants, whatever properties be selected as the diagnostic characters. There are many types perfectly distinct and others which intergrade. Some of the types change greatly with conditions while others do not. This is exactly what we encounter whenever we study the problem of species on an extended scale among the higher forms of life.