i8 



NATURE 



[September 3, 1914 



place in a progressive manner along definite and 

 divergent lines. 



(2) The fact that individual animals and plants are 

 more or less precisely adapted in their organisation 

 and in their behaviour to the conditions under which 

 they have to live. 



(3) The fact that evolution has resulted in the exist- 

 ence on the earth to-day of a vast number of more or 

 less well-defined groups of animals and plants which 

 we call species. 



The first of these facts appears to me to be the most 

 fundamental, and at the same time the one to which 

 least attention is usually paid. The great question, 

 after all, is, Why do organisms progress at all instead 

 of remaining stationary from generation to genera- 

 tion ? To answer this question it is not necessary to 

 go back to the beginning and consider the case of the 

 first terrestrial organisms, whatever they may have 

 been, nor are we obliged to take as illustrations the 

 lowest organisms known to us as existing at the 

 present day. We may consider the problem at any 

 stage of evolution, for at each stage progress is, or 

 may be, still taking place. We may even begin by 

 considering what is usually regarded as the highest 

 stage of all, man himself ; and indeed this seems the 

 most natural thing to do, for we certainly know more 

 about the conditions of progress in man than in any 

 other organism. I refer, of course, at the moment, 

 not to progress in bodily organisation, but to progress 

 in the ordinary sense of the word, the progress, say, 

 of a family which rises in the course of a fevv^ genera- 

 fions from a position of obscure poverty to one of 

 wealth and influence. You may perhaps say that such 

 a case has no bearing upon the problem of organic 

 evolution in a state of nature, and that we ought to 

 confine our attention to the evolution of bodily struc- 

 ture and function. If so, I must reply that you have 

 no right to limit the meaning of the term evolution in 

 this manner; the contrast between man and nature is 

 purely arbitrary; man is himself a living organism, 

 and all the improvements that he effects in his ow-n 

 condition are part of the progress of evolution in his 

 particular case. At any rate I must ask you to accept 

 this case as our first illustration of a principle that 

 may be applied to organisms in general. 



If we inquire into the cause of the progress of our 

 human family I think there can be only one answer — - 

 it is due to the accumulation of capital, or, as I should 

 prefer to put it, to the accumulation of potential 

 energ)', either in the form of material wealth or of 

 education. What one generation saves is available 

 for the next, and thus each succeeding generation gets 

 a better start in life, and is able to rise a little higher 

 than the preceding one. 



Every biologist knows, of course, that there are 

 many analogous cases amongst the lower animals, 

 and also amongst plants. The accumulation of food- 

 yolk in the egg has undoubtedly be'en one of the chief 

 factors in the progressive evolution of animals, 

 although it has been replaced in the highest forms by 

 a more effective method of supplying potential energy 

 to the developing offspring. It may indeed be laid 

 down as a general law that each generation, w^hether 

 of animals or of plants, accumulates more energy than 

 it requires for its own maintenance, and uses the 

 surplus to give the next generation a start in- life. 

 There is every reason to believe that this has been a 

 progressive process throughout the whole course of 

 evolution, for the higher the degree or organisation 

 the more perfect do we find the arrangements for 

 securing the welfare of the offspring. 



We cannot, of course, trace this process back to its 

 commencement, because we know nothing of the 

 nature of the earliest living things, but we may pause 



NO. 2340, VOL. 94] 



for a moment to inquire whether any phenomena occur 

 amongst simple unicellular organisms that throw any 

 light upon the subject. What we want to know is — 

 How did the habit of accumulating surplus energy and 

 handing it on to the next generation first arise? 



Students of Prof. H. S. Jennings's admirable work 

 on the "Behaviour of the Lower Organisms" will 

 remember that his experiments have led him to the 

 conclusion that certain Protozoa, such as Stentor, are 

 able to learn by experience how to make prompt and 

 effective responses to certain stimuli ; that after they 

 have been stimulated in the same way a number of 

 times they make the appropriate response at once 

 without having to go through the whole process of 

 trial and error by which it was first attained. In 

 other words, they are able b)- practice to perform a 

 given action with less expenditure of energy. Some 

 modification of the protoplasm must take place which 

 renders the performance of an act the easier the 

 oftener it has been repeated. The same is, of course, 

 true in the case of the higher animals, and we express 

 the fact most simply by saying that the animal estab- 

 lishes habits. From the mechanistic point of view 

 we might say that the use of the machine renders it 

 more perfect and better adapted for its purpose. In 

 the present state of our knowledge I think we cannot 

 go beyond this, but must content ourselves with recog- 

 nising the pow'cr of profiting by experience as a funda- 

 mental property of living protoplasm. 



It appears to me that this power of profiting by 

 experience lies at the root of our problem, and that 

 in it we find a chief cause of progressive evolution. 

 Jennings speaks of the principle involved here as the 

 " Law of the readier resolution of physiological states 

 after repetition," and, similarly, I think we must 

 recognise a " Law of the accumulation of surplus 

 energy " as resulting therefrom. Let us look at the 

 case of the accumulation of food-yolk by the egg-cell 

 a little more closely from this point of view. Every 

 cell takes in a certain amount of potential energy in 

 the form of food for its own use. If it leads an active 

 life, either as an independent organism or as a con- 

 stituent part of an organism, it may expend by far the 

 greater part, possibly even the whole, of that energy 

 upon its own requirements, but usually something is 

 left over to be handed down to its immediate descend- 

 ants. If, on the other hand, the cells exhibits very 

 little activity and expends very little energy, while 

 placed in an environment in which food is abundant, 

 it will tend to accumulate surplus energy in excess o| 

 its own needs. Such is the case with the egg-cells 

 of the multicellular animals and plants. Moreover, 

 the oftener the process of absorbing food-material is 

 repeated the easier does it become ; in fact, the egg- 

 cell establishes a habit of storing up reserve material 

 or food-yolk. Inasmuch as it is a blastogenic char- 

 acter, there can be no objection to the supposition that 

 this habit will be inherited by future generations of 

 egg-cells. Indeed we are obliged to assume that this 

 will be the case, for we know that the protoplasm of 

 each succeeding generation of egg-cells is directly con- 

 tinuous with that of the preceding generation. We 

 thus get at any rate a possibility of the progressive 

 accumulation of potential energv in the germ-cells of 

 successive generations of multicellular organisms, and, 

 of course, the same argument holds good with regard 

 to successive generations of Protista. 



It would seem that progressive evolution must follow 

 as a necessary result of the law of the accumulation 

 of surplus energy in all cases w'here there is nothing 

 to counteract that law, for each generation gets a 

 better start than its predecessor, and is able to carry 

 on a little further its struggle for existence with the 

 environment. It may be said that this argument 



