388 TRANSACTIONS OF SECTION D. 
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, however, 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 accumulation of surplus energy and of the slightly increased facility in re- 
sponding to stimuli. The additional progress, 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 backslidings due to various causes. 
We can thus formulate a perfectly reasonable explanation of how it is that 
the egg first undergoes segmentation and then gives rise to a blastula resembling 
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 con- 
version of the hollow spherical protozoon colony into the ccelenterate type of 
organisation, 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 blastoccel 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 
composed is of course derived from the egg-cell continuously 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 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 re!ly inherits from its parents two things : 
(1) a certain amount of protoplasm lozded with potential energy, with which to 
begin operations, and (2) an appropriate environment. Obviously the one is useless 
without the other. An egg cannot develop unless it is provided with the proper 
environment at every stage. Therefore, when we say that an organism inherits a 
particular character 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 adult, nearly the whole of which is captured from the environment and 
assimilated during ontogeny. 
From this point of view the distinction between somatogenic and blastogenic 
characters really disappears, for all the characters of the adult organism are 
acquired afresh in each generation as a result of response to environmental 
stimuli during development. 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 ccelen- 
terate passed into the celomate type, the ceelomate 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, however, about the influence of 
the environment, both upon the developing embryo and upon the adult, is suffi- 
* Quoted by Przibram, Hxperimental Zoology, English Trans., Part I., p. 47. 
“Compare Dr. Archdall Reid’s suggestive essay on ‘ Biological Terms’ 
(Bedrock, January 1914). 
