September 3, 1914] 



NATURE 



19 



proves too much, that if it were correct all organisms 

 would by this time have attained to a high degree of 

 organisation, and that at any rate we should not expect 

 to find such simple organisms as bacteria and Amoebae 

 still surviving. This objection, which, of course, 

 applies equally to other theories of organic evolution, 

 falls to the ground when we consider that there must 

 be many factors of which we know nothing which 

 may prevent the establishment of progressive habits 

 and render impossible the accumulation of surplus 

 energy. Many of the lower organisms, like many 

 human beings, appear to have an inherent incapacity 

 for progress, though it may be quite impossible for 

 us to say to what that incapacity is due. 



It will be observed that in the foregoing remarks I 

 have concentrated attention upon the storing up of 

 reserve material by the egg-cells, and in so doing have 

 avoided the troublesome question of the inheritance 

 of so-called acquired characters. I do not wish it to 

 be supposed, however, that I regard this as the only 

 direction in which the law of the accumulation of 

 surplus energy can manifest itself, for I believe that 

 the accumulation of surplus energy by the body may 

 be quite as important as a factor in progressive evolu- 

 tion as the corresponding process in the germ-cells 

 themselves. The parents, in the case of the higher 

 animals, may supply surplus energv, in the form of 

 nutriment or otherwise, to the offspring at all stages 

 of its development, and the more capital the young 

 animal receives the better will be its chances in life, 

 and the better those of its own offspring. 



In all these processes, no doubt, natural select^ion 

 plays an important part, but, in dealing with the 

 accumulation of food material by the egg-cells, one 

 of my objects has been to show that progressive 

 evolution would take place even if there were no such 

 thing as natural selection, that the slow successive 

 variations in this case are not chance variations, but 

 due to a fundamental property of living protoplasm 

 and necessarily cumulative. 



Moreover, the accumulation of surplus energy in the 

 form of food-yolk is only one of many habits which 

 the protoplasm of the germ-cells may acquire in a 

 cumulative manner. It may learn by practice to re- 

 spond with increased promptitude and precision to 

 other stimuli besides that of the presence of nutrient 

 material in its environment. It may learn to secrete 

 a protective membrane, to respond in a particular 

 manner to the presence of a germ-cell of the opposite 

 sex, and to divide in a particular manner after fer- 

 tilisation has taken place. 



Having thus endeavoured to account for the fact 

 that progressive evolution actually occurs by attribut- 

 ing it primarily to the power possessed by living proto- 

 plasm of learning by experience and thus establishing 

 habits by which it is able to respond more quickly to 

 environmental stimuli, we have next to inquire what 

 it is that determines the definite lines along which 

 progress manifests itself. 



Let us select one of these lines and investigate it 

 as fully as the time at our disposal will permit, with 

 the view of seeing whether it is possible to formulate a 

 reasonable hypothesis as to how evolution may hav^ 

 taken place. Let us take the line which we believe 

 has led up to the evolution of air-breathing verte- 

 brates. The only direct evidence at our disposal in 

 such a case is, of course, the evidence of palaeontology, 

 but I am going to ask you to allow me to set this 

 evidence, which, as you know, is of an extremely 

 fragmentary character, aside, and base my remarks 

 upon the ontogenetic evidence, which, although in- 

 direct, will, I think, be found sufficient for our pur- 

 pose. One reason for concentrating our attention 

 upon this aspect of the problem is that I wish to show 

 that the recapitulation of phylogenetic history in indi- 



NO. 2340, VOL. 94] 



vidual development is a logical necessity if evolution 

 has really taken place. 



We may legitimately take the nucleated Protozoon 

 cell as our starting point, for, whatever may have been 

 the course of evolution that led up to the cell, there 

 can be no question that all the higher organisms 

 actually start life in this condition. 



We suppose, then, that our ancestral Protozoon 

 acquired the habit of taking in food material in excess 

 of its own requirements, and of dividing into two parts 

 whenever it reached a certain maximum size. Here 

 again we must, for the sake of simplicity, ignore the 

 facts that even a Protozoon is by no means a simple 

 organism, and that its division, usually at any rate, 

 is a very complicated process. Each of the daughter- 

 cells presently separates from its sister-cell and goes its 

 own way as a complete individual, still a Protozoon. 

 It seems not improbable that the separation may be 

 due to the renewed stimulus of hunger, impelling each 

 cell to wander activeh- in search of food. In some 

 cases, however, the daughter-cells remain together 

 and form a colony, and probably this habit has been 

 rendered possible by a sufficient accumulation of surplus 

 energy in the form of food-yolk on the part of the 

 parent rendering it unnecessary for the daughter-cells 

 to separate in search of food at such an early date. 

 One of the forms of colony met with amongst existing 

 Protozoa is the hollow sphere, as we see it, for 

 example, in Sphaerozoum and Volvox, and it is highly 

 probable that the assumption of this form is due 

 largely, if not entirely, to what are commonly called 

 mathematical causes, though we are not in a jxjsition 

 to say exactly what these causes may be. The wide- 

 spread occurrence of the blastosphere or blastula stage 

 in ontogeny is a sufficiently clear indication that the 

 hollow, spherical Protozoon colony formed a stage in 

 the evolution of the higher animals. 



By the time our ancestral organism has reached 

 this stage, and possibly even before, a new complica- 

 tion has arisen. The cells of which the colony is com- 

 posed no longer remain all alike, but become diflFer- 

 entiated, primarily into two groups, which we distin- 

 guish as somatic-cells and germ-cells respectivelv. 



From this point onwards evolution ceases to be a 

 really continuous process, but is broken up into a 

 series of ontogenies, at the close of each of which the 

 organism has to go back and make a fresh start in 

 the unicellular condition, for the somatic cells sooner 

 or later become exhausted in their conflict with the 

 environment and perish, leaving the germ-cells be- 

 hind to take up the running. That the germ-cells do 

 not share the fate of the somatic cells must be attri- 

 buted to the fact that they take no part in the 

 struggle for existence to which the body is exposed. 

 They simply multiply and absorb nutriment under the 

 protection of the body, and therefore retain their 

 potential energ\- unimf>aired. They are in actual fact, 

 as is so often said, equivalent to so many protozoa, 

 and, like the protozoa, are endowed with a potential 

 immortalit}'. 



We know that, if placed under suitable conditions, 

 or in other words, if exposed to the proper environ- 

 mental stimuli, these germ-cells will give rise to new 

 organisms, like that in the body of which they were 

 formerly enclosed. One of the necessary conditions 

 is, with rare exceptions, the union of the germ-ceils 

 in pairs to form zygotes or fertilised ova ; but I pro- 

 pose, in the first instance, for the sake of simplicit\-. 

 to leave out of account the existence of the sexual 

 process and the results that follow therefrom, post- 

 poning the consideration of these to a later stage of 

 our inquirv'. I wish, moreover, to make it quite clear 

 that organic evolution must have taken place if no 

 such event as amphimixis had ever occurred. 



WTiat, then, may the germ-cells be exf>ected to do? 



