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 mani- 

 fest 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, espe- 

 cially 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 perma- 

 nent is, from our point of view, not surprising; but the fact that 

 in many cases the full virulence can by suitable cultivation be 



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