FUNCTION or EBSPIEATION. 199 



Coucludiag remarks. — ^With regard to the selective influence 

 of air and of the matters held in it — oxygen, water, carbonic acid, 

 &c. — the same general conclusion may evidently again be drawn 

 as we arrived at in the former chapters ; it results from the fact 

 that different animals or even individuals never react in pre- 

 cisely the same way under the influences of the atmosphere. 

 Every change in its composition must therefore essentially 

 alter the fauna of a country or of a locality by selection, if this 

 change is not merely a transitory one — in which case an inju- 

 rious or favourable influence may find compensation — but is 

 continuous for a lengthened period. A conflict between the 

 individuals thus affected need no more take place under the 

 selection caused by the conditions we have now been investiga- 

 ting than under any we have hitherto discussed. Such a 

 struggle can arise from this cause only when, from a super- 

 abundance of animals, the quantity of air at their disposal for 

 respiration has to be so greatly subdivided, that it fails to be 

 equally favourable to all the individuals. But while in all the 

 former chapters we recognised not selective effects only, but 

 also with more or less success a direct transforming influence 

 as exerted by the conditions under discussion, and even could 

 sometimes experimentally prove their existence, this has not 

 been the case as to air. The proof that a change in the func- 

 tion of respiration is possible may indeed be acquired by ex- 

 periment, and a not insignificant number of differences in the 

 structure of the respiratory organs concerned may be very 

 naturally conceived of as an immediate consequence of such a 

 chaiige of function ; but in no single case have we as yet suc- 

 ceeded in proving that .such a change of function as is involved 

 in the transformation of a gill-cavity into a lung must neces- 

 sarily be accompanied by definite changes in the structure of 

 that organ. Still it must not be forgotten that in this respect 

 we are not yet past the stage of the most superficial and 

 elementarv knowledge. 



