552 ALEXANDER PEDLER LECTURE 



determinants of evolution lies in the dual nature of life, expressed in the 

 legacy of heredity and the impact of environment. Opinions differ 

 widely as to the relative importance of these twin influences, but there 

 is no room for doubt that both exist, and that they may often prove 

 incompatible. 



An unfortunate but inevitable weakness of palaeontological evidence 

 enables it to show very little of the early history of groups of organisms, 

 although its record of their decline and fall is often clearly displayed. 

 We are far from knowing how or Vi'h}'' new types appear ; but on the 

 other hand we have plentiful illustration of how they disappear, and 

 convincing indication as to the way in which Nemesis overtakes them. 



The record of Evolution is, in essentials, the same for all groups of 

 organisms. Indeed, it is the same when expressed in the changes that 

 befall the several organs of which organisms are built. Phylogeny and 

 morphogeny are mutually dependent, for the whole, though greater than 

 the parts, consists of them and is directly affected by their condition. 

 Hyperbolic though it may sound, it is a bare fact of experience that the 

 life-story of an individual, or of a single cell in its body, is a precis of that 

 of a phylum, or of any taxonomic grade. Families and orders, like species 

 and individuals, may possess the contrasted qualities of ' perennials ' or 

 ' annuals ' ; but the general trend of their lives is the same. They have 

 their youth, a stage of growth and adaptation ; their maturity, when 

 equilibrium has been attained ; and their senility, when persistent de- 

 velopment beyond perfection leads to decline and death. 



In the youthful stage groups or individuals are plastic, producing much 

 diversity by the reaction of their intrinsic vitality with the moulding 

 influence of environment. In the senile stage their characters have become 

 stereotyped, and their reaction to an ever-changing environment is 

 extinction. The same inexorable range of variation in physical surround- 

 ings acts as a tonic to the young and a poison to the old. For life is a 

 competition between the mysterious quality called ' vitality ' and the 

 insensitive environment that encompasses it. The struggle is exhilarating, 

 creative, and usually successful, in youth ; but old age fights a losing 

 battle. The secret of perpetual youth is no mystery, for all that is needed 

 is perpetual plasticity, giving ready adaptation to environment. But in 

 the nature of things this is impossible. It is true that simple forms of life 

 can adjust themselves and their needs to varying conditions more readily 

 than more complex forms ; they have a greater expectation of racial life ; 

 but there is a term to their duration. Life itself, transmitted from one 

 generation to another, may be everlasting ; but all living things are mortal. 



This sounds like a somewhat morbid summary of the course of an 

 ordinary human life ; but actually it is a description of the ©volution of 

 every large or small group of organisms of which we have adequate 

 palaeontological knowledge. 



There are two harmonies essential for successful living, one internal 

 and the other external ; both must be kept consonant. The several organs 

 of an organism must maintain their proper proportions, and the organism 

 as a whole must conform to its surroundings. Internal discord, due to 

 the modulation of one ingredient independently of the rest, cannot fail 

 to produce inefliciency and final collapse ; while external discord brings 



