{ Ixxwi ; 



speculation to formulate the idea into a working hypothesis 

 worthy of the attention both of systematists and of physiolo- 

 gists. One of the most promising lines of advancement in the 

 future is in that field where these tAvo classes of workers will 

 co-operate. A large amount of observation is on record which 

 will bear interpretation by this hypothesis, but much of it 

 requires confirmation, and ail is more or less indefinite. Whole 

 chapters of facts which have been accumulated by writers on 

 evolution since Darwin's time might profitably be recon- 

 structed from this point of view. The nearest approach to 

 the systematic treatment of physiological correlation that I 

 am acquainted with is to be found in H. de Varigny's 

 little work on " Experimental Evolution,"* but even this 

 author does not make any attempt to co-ordinate his results 

 in the light of the selection theory. 



The experimental and observational verification of the 

 hypothesis is obviously surrounded by immense practical 

 difficulties — the difficulties naturally arising from the com- 

 plexity of the phenomena presented by living organisms, the 

 imperfection of our resources, the obstacles in the way of 

 disentangling one particular factor out of a multitude, and so 

 forth. None the less does it appear to me of the greatest 

 importance that the problem should be seriously attacked by 

 any and every means in our power.f The facts themselves are 



* Lectures delivered in the University Hall, Edinbnrgh, 1891. " Natni-e 

 Series," 18U2. Many of the observations in Prof. Karl Samper's work on 

 " Animal Life," " Iiiternat. Sci. Series," are also worthy of being recon- 

 sidered in connection with physiological correlation, as well as certain 

 chapters in Eimer's " Organic Evolution," Cunningham's Translation. 



+ Prof. H. F. Osborn, in the lecture referred to, argues iu favour of new 

 and unknown factors in organic evolution on the ground that the old factors 

 are inadequate. He says (loc. cit.,p. 82) : — " We are far from finally testing 

 or dismissing these old factors, but the reaction from speculation upon them 

 is in itself a silent admission that we may reach out for some unknown 

 quantity. If such does exist there is little hope that we shall discover it except 

 by the most laborious research ; and while we may predict that conclusive 

 evidence of its existence will be found in morphology, it is safe to add that 

 the fortunate discoverer will be a physiologist " The only point on which I 

 should differ from my friend, Prof. Osborn, is that the " unknown quantity " 

 should necessarily bo outside the pale of Darwinism. Physiological charac- 

 ters must be as subject to natural selection as morphological characters ; 

 physiological fitness must be as important an element iu determining survival 

 as morphological fitness. 



