386 TRANSACTIONS OF SECTION D. 
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 ameebe 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 atten- 
tion 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 evolution as the corre- 
sponding process in the germ-cells themselves. The parents, in the case of the 
higher animals, may supply surplus energy, in the form of nutriment or other- 
wise, 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 selection 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 respond 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 mem- 
brane, 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 fertilisation has taken 
place. 
Having thus endeavoured to account for the fact that progressive evolution 
actually occurs by attributing it primarily to the power possessed by living 
protoplasm 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 a view to seeing whether it is possible to formulate a 
reasonable hypothesis as to how evolution may have 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 paleontology, but [ 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 indirect, 
will, I think, be found sufficient for our purpose. 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 individual 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 
