24 president's address. 



identity underlying manifold differences — is undoubtedly that of 

 relationship by common descent, and no mere ideal of unity of 

 type. And this is the case quite independently of the question 

 whether or not the " modern doctrines of relationship by descent,, 

 heredity, and gradual differentiation of species by natural 

 selection have fui-nished a key to" a physico-chemical interpretation 

 of life. On the vitalistic view, of course these doctrines must be 

 held to represent no advance along the line of such interpretation. 

 Dr. Haldane holds that " the doctrine of natural selection does 

 not in any way offer a physico-chemical explanation of the means 

 by which the morphological and phj'^siological characters of an 

 organism are modified." Now this is just what it appears to me 

 natural selection does offer, so far as it goes. It is an attempt to 

 explain the facts of the admitted evolution of organic forms as a 

 series of events linked together by purely causal connection. 

 Last year I insisted upon its inadequacy as a complete principle 

 of explanation on account of its fundamental assumption of 

 (unexplained) variability. But that it is, nevertheless, an 

 actually operative factor in development, through whose use we 

 may be said to make progress in the recognition of the causal 

 sequences in biological phenomena, I can see no reason for 

 doubting. Yet no more here than anywhere else are we exempt 

 from the inevitable re-interpretation of all such phenomena, 

 when the causal principle is assigned its rightful place in a true 

 theory of knowledge as an abstract and incomplete principle of 

 interpretation. 



It is also true that for a complete analysis of the facts of 

 morphology we urgently require a tenable theory of heredity. 

 And it is objected that " no attempt worthy of serious considera- 

 tion has ever been made to furnish even the outlines of a physico- 

 chemical theory of heredity." 



The question of heredity is obviously bound up with that of 

 the structure and properties of the living matter which is carried 

 over from parent to offspring. Everyone must admit that the 

 substance of the oosperm is in some sense the embodiment and 

 the carrier of the characteristics of the parent organisms. As a 



