VARIATION 27 



cases in which new heritable characters have been artificially produced 

 by carefully controlled external stimuli. Hence some germinal variations 

 are apparently caused by known environmental conditions and we are 

 justified in recognizing this third category of developmental differences 

 due to environmental effects. 



Considerable evidence of permanent changes in both morphological 

 and physiological characters has been secured from experiments with the 

 culture of bacteria and yeast, in unusual culture media, in the presence 

 of toxic solutions, or under extreme temperature conditions. The sig- 

 nificant results of four investigators who worked independently, Hansen, 

 Barber, Wolf and Jordan, have been reviewed and discussed in regard to 

 their bearing on genetic theory by Cole and Wright. The four investi- 

 gators mentioned above used refined methods and three of them began 

 by isolating a single organism from whose progeny they obtained dis- 



O 



FIG. 11. O, Portion of leaf of parental Scrophularia showing branching lateral vein; 

 D, branching vein replaced by two laterals in leaf of a seedling grown from seed produced by 

 an injected ovary. Also note difference in size and margin of leaves. (After Mac Dougal.) 



tinct strains or biotypes which remained constant for hundreds of test- 

 tube "generations." It must be admitted that in most of these cases 

 no specific influence can be named as the direct cause of the inherited 

 variation. But there is no longer any doubt that permanent, discon- 

 tinuous variations do occur spontaneously in these lowest organisms, and 

 it is highly probable that certain incidental, external forces play an im- 

 portant part in inducing such variations. 



Direct experimental attack upon the germ cells themselves has 

 been made with plants by a number of investigators, notably by Mac- 

 Dougal, who injected very dilute solutions of potassium iodide, zinc 

 sulphate, sugar, etc., directly into the ovaries of various plants imme- 

 diately before fertilization. Consequently somatic changes have been 

 produced which were inherited throughout several generations. By 

 means of check experiments and observations it was found that these 

 germinal variations were not caused by the wounding of the ovary and it 

 is thought that they must have been induced in some way by the presence 

 of the foreign chemical solution in the ovary. Fig. 11 shows a mor- 

 phological change which appeared in a seedling of an unnamed species 



