20 



NATURE 



[September 3, 1914 



How are they going to begin their development? In 

 endeavouring to answer this question we must re- 

 member that the behaviour of an organism at any 

 moment depends upon two sets of factors — the nature 

 of its own constitution on one hand, and the 

 nature of its environment on the otlier. If these 

 factors are identical for any two individual organisms, 

 then the behaviour of these two individuals must be 

 the same. If the germ-cells of any generation are 

 identical with those of the preceding generation, and 

 if they develop under identical conditions, then the 

 soma of the one generation must also be identical 

 with that of the other.- Inasmuch as they are parts 

 of the same continuous germ-plasm — leaving out of 

 account the complications introduced by amphimixis — 

 we may assume that the germ-cells of the two genera- 

 tions are indeed identical in nearly every respect ; 

 but there will be a slight difference, due to the fact 

 that those of the later generation will have inherited 

 a rather larger supply of initial energy and a slightly 

 greater facility for responding to stimuli of various 

 kinds, for the gradual accumulation of these proper- 

 ties will have gone a stage further. The environment 

 also will be very nearly identical in the two cases, for 

 we know from experiment that if it were not the 

 organism could not develop at all. 



Throughout the whole course of its ontogeny the 

 organism must repeat with approximate accuracy the 

 stages passed through by its ancestors, because at 

 every stage there will be an almost identical organism 

 exposed to almost identical stimuli. We may, how- 

 ever, expect an acceleration of development and a 

 slight additional progress at the end of ontogeny as 

 the result of the operation of the law of the accumula- 

 tion of surplus energy and of the slightly increased 

 facility in responding to stimuli. The additional pro- 

 gress, of course, will probably be so slight that from 

 one generation to the next we should be quite unable 

 to detect it, and doubtless there will be frequent back- 

 slidings due to various causes. 



We can thus formulate a perfectly reasonable ex- 

 planation of how it is that the ^^^ first undergoes 

 segmentation and then gives rise to a blastula resem- 

 bling a hollow protozoon colony ; it does so simply 

 because at every stage it must do what its ancestors 

 did under like conditions. We can also see that 

 progressive evolution must follow from the gradual 

 accumulation of additions at the end of each ontogeny, 

 these additions being rendered possible by the better 

 start which each individual gets at the commencement 

 of its career. 



Let us now glance for a moment at the next stage 

 in phylogeny, the conversion of the hollow spherical 

 protozoon colony into the coelenterate type of organ- 

 isation, represented in ontogeny by the process of 

 gastrulation. Here again it is probable that this 

 process is explicable to a large extent upon mechanical 

 principles. According to Rhumbler,' the migration 

 of endoderm cells into the interior of the blastula is 

 partly due to chemotaxis and partly to changes of 

 surface tension, which decreases on the inner side 

 of the vegetative cells owing to chemical changes set 

 up in the blastocoel fluid. 



We may, at this point, profitably ask the question, 

 Is the endoderm thus formed an inherited feature of 

 the organism? The material of which it is com- 

 posed is, of course, derived from the egg-cell con- 

 tinuously by repeated cell-division, but the way in which 

 that material is used by the organism depends upon 

 the environment, and we know from experiment that 

 modifications of the environment actually do produce 



^ This is, of course, a familiar idea. Compare Driesch, "Gifford Lectures," 

 1907, p. 214. 



s Quoted by Przibram, " Experimental Zoology," English Trans., Parti., 

 P- 47- 



NO. 2340, VOL. 94] 



corresponding modifications in the arrangement of the 

 material. We know, for example, that the addition 

 of salts of lithium to the water in which certain 

 embryos are developing causes the endoderm to be 

 protruded instead of invaginated, so that we get a 

 kind of inside-out gastrula, the well-known lithium 

 larva. 



It appears, then that an organism really inherits 

 from its parents two things : (i) a certain amount of 

 protoplasm loaded with potential energy, with which 

 to begin operations, and (2) an appropriate environ- 

 ment. Obviously the one is useless without the other. 

 An &g^ cannot develop unless it is provided with the 

 proper environment at every stage. Therefore, when 

 we say that an organism inherits a particular char- 

 acter from its parents, all we mean is that it inherits 

 the power to produce that character under the influence 

 of certain environmental stimuli.* The inheritance 

 of the environment is of at least as much importance 

 as the inheritance of the material of which the 

 organism is composed. The latter, indeed, is only 

 inherited to a very small extent, for the amount of 

 material in the egg-cell may be almost infinitesimal 

 in comparison with the amount present in the aduk, 

 nearly the whole of which is captured from the en- 

 vironment and assimilated during ontogeny. 



From this point of view the distinction between 

 somatogenic and blastogenic characters really dis- 

 appears, for all the characters of the adult organism 

 are acquired afresh in each generation as a result of 

 response to environmental stimuli during develop- 

 ment. This is clearly indicated by the fact that you 

 cannot change the stimuli without changing the 

 result. 



Time forbids us to discuss the phylogenetic stages 

 through which the coelenterate passed into the ccelo- 

 mate type, the coelomate into the chordate, and the 

 chordate into the primitive vertebrate. We must 

 admit that as yet we know nothing of the particular 

 causes that determined the actual course of evolution 

 at each successive stage. What we do know, how- 

 ever, about the influence of the environment, both 

 upon the developing embryo and upon the adult, is 

 sufficient to justify us in believing that every succes- 

 sive modification must have been due to a response 

 on the part of the organism to some environmental 

 change. Even if the external conditions remained 

 practically identical throughout long periods of time, 

 we must remember that the internal conditions would 

 be different in each generation, because each genera- 

 tion starts with a slightly increased capital and carries 

 on its development a little further under internal 

 conditions modified accordingly. 



At this point it may be asked. Is the response to 

 environmental stimuli a purely mechanical one, and. 

 if so, how can we account for the fact that at every 

 stage in its evolution the organism is adapted to its 

 environment? We shall have to return to this ques- 

 tion later on, but it may be useful to point out once 

 more that there is good reason to believe — especially 

 from the experimental work of Jennings — that the 

 response of even a unicellular organism to stimuli is 

 to a large extent purposive; that the organism learns 

 by experience, by a kind of process of trial and error, 

 how to make the response most favourable to itself 

 under any given change of conditions ; in other words, 

 that the organism selects those modes of response that 

 are most conducive to its own well-being. L'nder the 

 term response to stimuli we must, of course, include 

 those responses of the living protoplasm which result 

 in modifications of bodily structure, and hence the 

 evolution of bodily structure will, on the whole, be 



* Compare Dr. Archdall Reid's suggestive essay on "Biological Terms" 

 {Bedrock, January, 1914). 



