394 TRANSACTIONS OF SECTION D. 
put forward is clearly epigenetic in character, while the theory of unit characters, 
represented in the germ-cells by separate ‘factors,’ is hardly less clearly a 
theory of preformation, and of course the conception of definite organ-forming 
substances in the cytoplasm falls under the same category. The point which 
I now wish to emphasise is that the ideas of epigenesis and preformation are not 
inconsistent with one another, and that, as a matter of fact, ontogenetic develop- 
ment is of a dual nature, an epigenesis modified by what is essentially pre- 
formation. 
We have already dealt briefly with the question of organ-forming substances 
in the cytoplasm, and it must, I think, be clear that the existence of these is in 
no way incompatible with a fundamental epigenesis. We shall find directly 
that the same is true of Mendelian ‘ factors’ or Weismannian ‘ determinants.’ 
We have seen that it is possible to conceive of even a complex organism as 
inheriting nothing from its parent but a minute speck of protoplasm, endowed 
with potential energy, and a sequence of suitable environments, the interaction 
between the two bringing about a similar result in each succeeding generation, 
with a slow progressive evolution due to the operation of the law of accumulation 
of surplus energy. If any of the conditions of development are changed the 
result, as manifested in the organisation of the adult, must undergo a correspond- 
ing modification. Suppose that the chromatin substance of the zygote is partially 
modified in molecular constitution, perhaps by the direct action of the environ- 
ment, as appears to happen in the case of Tower’s experiments on mutation in 
the potato beetle, or by the introduction of a different sample of chromatin from 
another individual by hybridisation. What is the germ-plasm now going to do? 
When and how may the changes that have taken place in its constitution be 
expected to manifest themselves in the developing organism? 
Let us consider what would be likely to happen in the first stages of 
ontogeny. If the germ-plasm had remained unaltered the zygote would have 
divided into blastomeres under the stimuli of the same conditions, both internal 
and external, as those under which the corresponding divisions took place in 
preceding generations. Is the presence of a number of new colloidal multi- 
molecules in the germ-plasm going to prevent this? The answer to this question 
probably depends partly upon the proportion that the new multimolecules bear 
to the whole mass, and partly upon the nature of the modification that has taken 
place. If the existence of the new multimolecules is incompatible with the 
proper functional activity of the germ-plasm as a whole there is an end of the 
matter. The organism does not develop. If it is not incompatible we must 
suppose that the zygote begins its development as before, but that sooner or later 
the modification of the germ-plasm will manifest itself in the developing 
organism, in the first instance as a mutation. In cases of hybridisation we may 
get a mixture in varying degrees of the distinguishing characters of the two 
parent forms, or we may get complete dominance of one form over the other 
in the hybrid generation, or we may even get some new form, the result depend- 
ing on the mutual reactions of the different constituents of the germ-plasm. 
The organism into which any zygote develops must be a composite body 
deriving its blastogenic characters from different sources; but this cannot affect 
its fundamental structure, for the two parents must have been alike in all 
essential respects or they could not have interbred, and any important differences 
in the germ-plasm must be confined to the ‘factors’ for the differentiating 
characters. The fundamental structure still develops epigenetically on the basis 
of an essentially similar germ-plasm and under essentially similar conditions as 
in the case of each of the two parents, and there is no reason to suppose that 
special ‘ factors ’ have anything to do with it. 
We thus see how new unit characters may be added by mutation and inter- 
changed by hybridisation while the fundamental constitution of the organism 
remains the same and the epigenetic course of development is not seriously 
affected. All characters that arise in this way must be regarded, from the 
point of view of the organism, as chance characters due to chance modifications 
of the germ-plasm, and they appear to have comparatively little influence upon 
the course of evolution. 
One of the most remarkable features of organic evolution is that it results 
in the adaptation of the organism to its environment, and for this adaptation 
