Remarks on the Nature and Definition of Species. 51 



relatively to that of other organisms. That is to say they in- 

 volve constructional limitations such that the range of factors, 

 material or dynamic, to which any given organism is capable 

 of responding is intrinsically limited. No possible method of 

 culture can change a bacillus directly into a Botrytis, but even 

 in reo:ard to the broad features which separate group from group, 

 the fundamentals of structirre or hypomorphs, as we may call 

 them, in distinction from allelomorphs — these factors of con- 

 stitution which rough-hew the generic and familial outlines of 

 the organism— it still remains to assess the full effect of en\'iron- 

 mental pressure acting over long periods of time. 



That which is passed from generation to generation is not 

 form but habit, the hxity of which is in direct proportion to its 

 pre\'ious duration ; and on the extent to which this is compatible 

 with a new en\Tronment \\ill depend the continued 5ur\-ival of 

 the organism. Habit being ine\itably of finite duration can 

 never attain to an infinite degree of fixity. 



Inseparable from the enquiry- into the possibility of physio- 

 logical change is that into educability, a postulate which has 

 been far too hghtly accepted. It must be admitted at this 

 present time, that much of the collected e\-idence supposed to 

 uphold the idea is exphcable on the ground of selection in 

 mixed populations and that experience with pure lines under 

 strictly standardized conditions renders it dubious. I have 

 already pointed out that the mass of e^■idence illustrating that 

 a given organism generally reacts to the same en\Tronment in 

 the same morphological manner offers no e\"idence on the 

 crucial point of the maintenance, throughout its history", of 

 an identical constitution, but only suffices to show that its 

 reaction in face of so and so is such and such, nothing more. 

 Even the invariabihty of the organism in this restricted sense 

 is, however, by no means certain and when we are offered 

 growth on potato or onion as examples of the rigidly standardized 

 conditions under which organismal reactions have been obsen-ed 

 by its upholders, we may be permitted, perhaps, to doubt %\hether 

 the alleged constancy rests on quite so sound a basis as we are 

 expected to believe. 



The converse problem, that of the nature of the alterations 

 induced by en\'ironmental change is likewise often prejudiced 

 by the presence of the same assumption of the self-existence of 

 the specific form. Brierley's clear analysis has done much to 

 resolve the issues and has emphasized the fallacious nature of 

 much of the e\-idence adduced, particularly among the bacteria ; 

 where mv own results in attempting to educate a pure line of 

 a spore bearing anaerobe into toleration of flaN^ne entirely bear 

 out his negative experiences with Botrytis cinerea. The behef 



4—2 



