260 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 39 



In many cases, and for various reasons, our apprehension of the 

 causes of evolutionary change is far less complete than in the case of 

 physical processes ; but the constant repetition of similar effects gives 

 presumptive evidence of oft-recurring cause. The chief difficulty in 

 appraising the determinants of evolution lies in the dual nature of 

 life, expressed in the legacy of heredity and the impact of environ- 

 ment. 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 paleontological evidence 

 enables it to show very little of the early history of groups of organ- 

 isms, although its record of their decline and fall is often clearly dis- 

 played. We are far from knowing how or why new types appear; 

 but on the other hand we have plentiful illustration of how they dis- 

 appear, 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 development 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 

 molding influence of environment. In the senile stage their char- 

 acters have become stereotyped, and their reaction to an ever changing 

 environment is extinction. The same inexorable range of variation 

 in physical surroundings 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 



