532 



SCIENCE 



[N. S. Vol. LIV. No. 1405. 



In an environment whicli on the -whole alters 

 but little, evolution progresses by the cumu- 

 lation along diverging lines of adaptation of 

 new characters due to mutation. Thus nat- 

 ural selection indirectly preserves those fac- 

 torial complexes which respond in a favorable 

 manner. In other words, ,in orijanism, to sur- 

 vive in the struggle for existence, must pre- 

 sent that assemblage of factors of inheritance 

 which, under the existing environmental condi- 

 tions, will give rise to advantageous char- 

 acters. 



In answer to a further question, let us 

 now try to explain what we mean when we 

 contrast the organism with its environment. 

 In its simplest and most abstract form a liv- 

 ing organism may be likened to a vortex. 

 That mixture of highly complex proteins we 

 call protoplasm, the physical basis of life, 

 is perpetually undergoing transformations of 

 matter and energy, so long as life persists. 

 Towards the center of the vortex the highest 

 compounds are continually being built up and 

 continually being broken down; new material 

 (food, water, oxygen) and energy are brought 

 in at the periphery, and old material and en- 

 ergy (work and heat) thrown out. The prin- 

 ciple of the conservation of energy and matter 

 holds good in organized living processes as 

 it does in the inorganic world outside. This 

 is the process we call metabolism, and it is at 

 the base of all the manifestations of life. 

 From the point of view of biological sci- 

 ence life is founded on a complex and con- 

 tinuous physico-chemical process of endless 

 duration so long as conditions are favorable; 

 just as a fire will continue to burn so long as 

 fuel is at hand. No one step, no single sub- 

 stance, can be said to be living; the whole 

 chain of substances and reactions, every liruk 

 of which is essential, constitutes the life- 

 process. A stream of non-living matter with 

 stored-up energy is built up into the living vor- 

 tex, and again passes out as dead ruatter, hav- 

 ing yielded up the energy necessary for the 

 performance of the various activities of the 

 organism. If more is taken in than is given 

 out it will grow and sub-divide. The com- 

 plexity of the organism may increase by the 



formation of subsidiary, more or less inter- 

 dependent, vortices within it. The perpetual 

 growth and transmission of factors of in- 

 heritance, the continuity of the germ-plasm, is 

 but another aspect of the continuity of the 

 metabolic process forming the basis of the 

 continuity of life in evolution. 



But all the environmental stimuli are not 

 external to the organism. Just as the va- 

 rious steps in the metabolic process are depen- 

 dent on those which preceded them, so when 

 an organism becomes differentiated into parts, 

 when the main process becomes subdivided 

 into subsidiary ones, these react on each other. 

 What is internal to the whole becomes ex- 

 ternal to the part. An external stimulus may 

 set up an internal metabolic change, giving 

 rise to a response whose extent and nature de- 

 pend on the structure of the mechanism and 

 its state when stimulated, that is to say, on 

 the effect of previous responses. Such a re- 

 sponse may act as an internal stimulus giving 

 rise to a further response, which may modify 

 the first, and so on. Parts thus become mar- 

 velously fitted to set going, inhibit, or regu- 

 late each other's action; and thus arises that 

 power of individual adaptation, or self-regula- 

 tion, so characteristic of living organisms. 

 The processes of temperature regulation, of 

 respiration, of excretion are examples of such 

 delicate self-regulating mechanisms in our- 

 selves. But one of the great advantages there- 

 by gained by organisms is that they can regu- 

 late their own growth and ensure their own 

 " right " development. "Whereas the simplest 

 plants and animals are to a great extent, so 

 to speak, at the mercy of their external en- 

 vironment, except in so far as they can move 

 from unfavorable to more favorable surround- 

 ings; whereas their characters appear in re- 

 sponse to external stimuli which may or may 

 not be present, and over which they have little 

 or no control — the higher organisms (more 

 especially the higher animals), as it were, grad^ 

 ually suhstitute internal for external stimuli. 

 Food material is provided in the ovum, and 

 the size, structure and time of appearance of 

 various characters are regulated to a great ex- 

 tent by use and by the secretions of various 



