November 24, 1921] 



NATURE 



405 



in the fertilised ovum from which it develops, 

 they are produced anew at every generation.* No 

 distinction in kind or value can be drawn between 

 characters. 



If some are inherited regularly and others are 

 not, the distinction lies not in the nature or mode 

 of production of the characters themselves, but in 

 the constancy of the factors and conditions which 

 give rise to them. Thus, although there is only- 

 one kind of character, there are two kinds of 

 variation. 



Much of the confusion in evolutionary litera- 

 ture is, I think, due to the use of the word varia- 

 tion in a loose manner. Sometimes it is taken to 

 mean the degree of divergence between two in- 

 dividuals ; sometimes the character itself in which 

 they differ, such as a colour or spot on a butter- 

 fly's wing; at other times a variety or race differ- 

 ing from the normal form of the species. If clear- 

 ness of thought and expression is to be attained, 

 the word variation should mean the extent or 

 degree of difference between two individuals or 

 between an individual and the average of the 

 species, the divergence of the new form from the 

 old ; not a new character or assemblage of char- 

 acters, but a difference which can be measured or 

 at least estimated. We shall then find that a 

 variation is of one of two kinds (which may, of 

 course, be combined) : the first kind is due to 

 some change in the complex of effective environ- 

 mental stimuli, the second to some change in 

 the complex of germinal factors. 



The second kind, to which the name mutation 

 has been applied, will, under constant conditions, 

 be inherited since the new complex of factors 

 will be transmitted to subsequent generations. 

 The first kind of variation, which has been called 

 a modification, will also be inherited, provided, 

 of course, the change of stimulus persists. In 

 either case, new characters will result. But here, 

 again, we must be careful not to apply the terms 

 nutation and modification to the characters them- 

 selves, as is so often done ^ ; for we then reintro- 

 duce the confusion already exposed in the popular 

 but misleading distinction between "acquired" 

 and "non-acquired" characters. The characters 

 due to mutation or modification are, of course, 

 indistinguishable by mere inspection, and can 

 only be separated by experiment. A mutation 

 once established should give rise, under uniform 

 conditions, to a new heritable character, and may 

 be detected by crossing with normal members of 

 the species. 



So far observations and tests have shown that 

 :ew characters due to modification only reappear 

 ^o long as the new stimulus persists. The differ- 

 ence lies not in the value or permanence of the 



♦ In other words, all characters are " acquired during the lifetime of the 

 individual," and "inherited" in the sense here defined has just the same 

 meaning. Much the same view was advocated by Prof. A. Sedgwick in his 

 address to this Section at D.-,ver in 1899, and it has also been developed by 

 Sir Archdall Raid and others. 



' The name " mutation " might be given to the alteration in the factors 

 instead of the variation due to it. The latter might then be termed a muta- 

 tional \-ariation and would be opposed to a modificational variation. At 

 present the term "mutation" is applied to three different things: the 

 aciorial change, the variation or difference, and the new product, respon se 

 for character. 



^x). 2717, VOL. 108] 



new character, but in the causes which give rise 

 to it.6 



It is little more than a platitude to state that 

 for the production of an organism or of any of 

 its characters both germinal factors and environ- 

 mental stimuli are necessary, and that if evolution 

 is to take place there must be change in one or 

 both. Yet the changes in the factors may be held 

 to be the more important. In an environment 

 which on the whole alters but little, evolution pro- 

 gresses by the cumulation along diverging lines 

 of adaptation of new characters due to mutation. 

 Thus natural selection indirectly preserves those 

 factorial complexes which respond in a favourable 

 manner. In other words, an organism, to survive 

 in the struggle for existence, must present that 

 assemblage of factors of inheritance which, under 

 the existing environmental conditions, will give 

 rise to advantageous characters. 



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 living organism may be 

 likened to a vortex. That mixture of highly com- 

 plex proteins we call protoplasm, the physical 

 basis of life, is perpetually undergoing trans- 

 formations of matter and energy, so long as life 

 persists. Towards the centre 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 energy 

 (work and heat) thrown out. The principle of the 

 conservation of energy and matter holds good in 

 organised 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 science life is founded on a complex 

 and continuous physico-chemical process of end- 

 less duration so long as conditions are favourable ; 

 just as a fire will continue to burn so long as fuel 

 is at hand. No one step, no single substance, 

 can be said to be living : the whole chain of sub- 

 stances and reactions, every link of which is 

 essential, constitutes the life-process. A stream 

 of non-living matter with stored-up energy is built 

 up into the living vortex, and again passes out as 

 dead matter, having yielded up the energy neces- 

 sary 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 complexity 

 of the organism may increase by the formation of 

 subsidiary, more or less interdependent, vortices 

 within it. The perpetual growth and transmission 

 of factors of inheritance, the continuity of the 

 germ-plasm, is but another aspect of the con- 

 tinuity of the metabolic process forming the basis 

 of the continuity of life in evolution. 



But all environmental stimuli are not external 



S We might perhaps distinguish the two cases by calling them constant 

 and inconstant characters, or "natural" and "acquired," as is commonly 

 done when de<cribinz immunity. It should be meant thereby that one is 

 acquired usually (under normal conditions), the other occasionally (when 

 infection occurs). Error creeps in when the term "acquired '" is opposed to 

 '■ non-acquired " or to " inherited." 



