34 GENERAL PRELIMINARY CONSIDERATIONS. 



co-operative and sometimes antagonistic, sometimes under the 

 influence of tlie external conditions of life and sometimes opposed 

 to them. 



Under the term ' Physiology of Organisms ' we may there- 

 fore comprehend all those laws which are known to us from the 

 investigation of the relations of vaiioua species to each other 

 and to those conditions of life which maintain, destroy, or 

 modify their existence as species. Special physiology, on the 

 other hand — which in its pi-esent stage is often termed simply 

 human physiology — or, more accurately, the 'Physiology of 

 organs,' includes all those facts and laws which refer chiefly 

 or exclusively to the specific action or re-action of the organs 

 of individuals. 



The subject matter of the following chapters has now been 

 exactly enough defined. "VVe shall leave the laws of the re- 

 lations of affinity as I'evealed by morphology entirely out of 

 the question, accepting them as they stand, without criticism. 

 In the province of physiology we shall in the same way dis- 

 regard, as far as possible, the physiology of organs ; for, at any 

 rate in the first instance, it is of no importance to our more 

 genera] problem that the use of each organ should be deter- 

 mined. The interest of these specific enquiries extends only 

 so 'far as they may be of value in determining for a species its 

 capability of existence as such. At the same time it must 

 never be forgotten that the results of the more general enquiry 

 can never contradict the really well-founded facts and laws of 

 special physiology, and we shall conseqxiently be obliged again 

 and again to refer to them, especially when a species depends 

 principally or exclusively on the healthy and vigorous action of 

 its organs for the possibility of maintRining its place in the 

 struggle for existence. 



Before going on to this particular enquiry it seems desirable 

 that the expression ' External Conditions of Existence ' should 

 be as accurately defined as may be. I have already said that I 

 wish to see as wide an application given to it as possible, so as 

 to include every influence, however insignificant and difiicult to 

 detect, that can affect the ' fitness for survival ' of a species, and 

 to investigate its mode of action. This explanation might 



