PRESIDENTIAL ADDRESS, 391 
very different from those under which gill-slits were developed in their aquatic 
ancestors. But what then? Are not the gill-slits also very different? The 
changed environment has had its effect. The gills themselves are never 
developed, and the gill-slits never become functional; moreover, they disappear 
completely at later stages of development, when the conditions of life become 
still more different and their presence would be actually detrimental to their 
possessor. The embryo with the vestigial gill-slits is, as a whole, perfectly well 
adapted to its environment, though the gill-slits themselves have ceased to be 
adaptive characters. They still appear because the environmental conditions, 
and especially the internal conditions, which have now become far more 
important than the external ones, are still such as to cause them to do so. 
I think the chief difficulty in forming a mental picture of the manner in 
which evolution has taken place, and especially in accounting for the phenomenon 
of recapitulation in ontogeny, which is merely another aspect of the same 
problem, arises from attempting to take in too much at once. There is no 
difficulty in understanding how any particular stage is related to the correspond- 
ing stage in the previous generation, and the whole series of stages, whether 
looked at from the ontogenetic or from the phylogenetic point of view, can be 
nothing else but the sum of its successive terms. 
It will be convenient, before going further, to sum up the results at which 
we have so far arrived from the point of view of the theory of heredity. We 
have as yet seen no reason to distinguish between somatogenic and blastogenic 
characters. All the characters of the adult animal are acquired during onto- 
geny as the result of the reaction of the organism to environmental stimuli, 
both internal and external. All that the organism actually inherits is a certain 
amount of protoplasm—endowed with a certain amount of energy—and a certain 
sequence of environmental conditions. In so far as these are identical in any 
two successive generations the final result must be identical also, the child must 
resemble the parent; in so far as they are different the child will differ from 
the parent, but the differences in environment cannot be very great without 
preventing development altogether. 
So far, it is clear, there has been no need to think of the germ-cells ag 
the bearers of material factors or determinants that are responsible for the 
appearance of particular characters in the adult organism; nor yet to suppose 
that they are, to use the phraseology of the mnemic theory of heredity, charged 
with the memories of past generations. They have been regarded as simple 
protoplasmic units, and the entire ontogeny has appeared as the necessary result 
of the reaction between the organism and its environment at each successive 
stage of development. This cannot, however, be a complete explanation of 
ontogeny, for if it were we should expect all eggs, when allowed to develop 
under the same conditions from start to finish, to give rise to the same adult 
form, and this we know is not the case. We know also, from observation and 
experiment, that the egg is in reality by no means a simple thing but an 
extremely complex one, and that different parts of the egg may be definitely 
correlated with corresponding parts of the adult body. It has been demon- 
strated in certain cases that the egg contains special organ-forming substances 
definitely located in the cytoplasm, and that if these are removed definite parts 
of the organism into which the egg develops will be missing. We know, also, 
that the nucleus of the germ-cell of either sex contains—at any rate at certain 
periods—a number of perfectly well-defined bodies, the chromosomes, and these 
also have been’ definitely correlated in certain cases with special features of the 
adult organisation. 
Before we can hope to complete our mental picture of the manner in which 
organic evolution has taken place, if only in outline, it is evident that we must 
be able to account for the great complexity of structure which the germ-cells 
themselves have managed to acquire, and also to form some idea of the effect of 
this complication upon the development of both the individual and the race. 
We must consider the origin of cytoplasmic and nuclear complications of the 
egg separately, for they appear to be due fundamentally to two totally distinct 
sets of factors. In the first place we have to remember that during oogenesis 
the egg-cell grows to a relatively large size by absorbing nutrient material from 
the body in which it is enclosed. 1t is this nutrient material that is used for 
