52 Transactions British Mycological Society. 



that an organism can be so remodelled by its contact with a 

 certain set of external factors that it refuses to return to its 

 original fashion when returned to its original environment is 

 unsupported by any conclusive evidence. It is, in any case, not 

 to be apprehended a priori that an organism which has once 

 clearly shown itself to be plastic by responding to a change of 

 conditions, should suddenly become aplastically fixed in a 

 new posture. Until such an eventuality be unequivocally de- 

 monstrated to occur we may safely leave it out of account in 

 our specific determinations. 



This negation is not to be regarded as excluding the possi- 

 bility of spontaneous mutation ; a kind of collapse — as we have 

 already described it — of the specific framework; but neverthe- 

 less the same caveat as before must be entered against this 

 proposition also. The manifold sequelae of heterozygosis must 

 not be overlooked ; which implies a more thorough scrutiny of 

 possibilities than has been carried out in the cases of most 

 observed "mutations." 



On the environmental hypothesis of specific form which is 

 here suggested, a pure mutation, should such a thing be shown 

 to occur, would be held to alter the capacity of the protoplasm 

 for response to the environmental factors, either in the direction 

 of diminishing or in extending its sensitiveness to include new 

 influences which previously did not perceptibly affect it. Within 

 the limits of such an extension of the range of factors, whether 

 climatic, edaphic or biotic, with which the organism is in sensi- 

 tive contact, evolution, judged thermodynamically, will have 

 advanced a material step through the mutation. 



There is, however, another aspect of educability, often 

 neglected, which suggests considerable possibilities when taken 

 side by side with the hypothesis of pure mutation. Although it 

 may not prove possible to alter experimentally the intrinsic 

 capacity of a pure line with respect to some new factor, yet it 

 is certainly possible to "acclimatise" even a pure strain to new 

 conditions by the well-known procedure of moving by slow 

 degrees. Instances of such occurrences are easy to procure at 

 first hand in the laboratory and are on record from almost all 

 groups of plants. Nothing, we may assume, has been per- 

 manently added to the constitution of the species by acclimatiza- 

 tion. We have only suceeded in straining its powers of response 

 to their utmost limit by avoiding destructive shock. There is 

 nothing specially vital in such a process, for mechanics is full 

 of similar effects. Thus, a wire will support a certain weight 

 added to it in small increments, while the same weight suddenly 

 applied breaks the wire. Again, a horizontal, suspended bar is 

 acted upon by a longitudinal pressure, slowly applied: the bar 



