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the lower animals, and also amongst plants. The accumulation of food-yolk in 
the egg has undoubtedly been one of the chief factors in the progressive evolu- 
tion of animals, although it has been replaced in the highest forms by a 
more effective method of supplying potential energy to the developing offspring. 
It may indeed be laid down as a general law that each generation, whether of 
animals or of plants, accumulates more energy than it requires for its own main- 
tenance, and uses the surplus to give the next generation a start in life. There 
is every reason to believe that this has been a progressive process throughout 
the whole course of evolution, for the higher the degree of organisation the more 
perfect do we find the arrangements for securing the welfare of the offspring. 
We cannot, of course, trace this process back to its commencement, because 
we know nothing of the nature of the earliest living things, but we may pause 
for a moment to inquire whether any phenomena occur amongst simple 
unicellular organisms that throw any light upon the subject. What we want to 
know is—How did the habit of accumulating surplus energy and handing it on 
to the next generation first arise? 
Students of Professor H. 8. Jennings’ admirable work on the ‘ Behaviour of 
the Lower Organisms ’ will remember that his experiments have led him to the 
conclusion that certain Protozoa, such as Stentor, are able to learn by experience 
how to make prompt and effective responses to certain stimuli; that after they 
have been stimulated in the same way a number of times they make the appro- 
priate response at once without having to go through the whole process of trial 
and error by which it was first attained. In other words, they are able by 
practice to perform a given action with less expenditure of energy. Some 
modification of the protoplasm must take place which renders the performance 
of an act the easier the oftener it has been repeated. The same is of course 
true in the case of the higher animals, and we express the fact most simply by 
saying that the animal establishes habits. From the mechanistic point of view 
we might say that the use of the machine renders it more perfect and better 
adapted for its purpose. In the present state of our knowledge I think we 
cannot go beyond this, but must content ourselves with recognising the power of 
profiting by experience as a fundamental property of living protoplasm. 
It appears to me that this power of profiting by experience lies at the root of our 
problem, and that in it we find a chief cause of progressive evolution. Jennings 
speaks of the principle involved here as the ‘ Law of the readier resolution of 
physiological states after repetition,’ and, similarly, I think we must recognise 
a ‘Law of the accumulation of surplus energy ’ as resulting therefrom. Let us 
look at the case of the accumulation of food-yolk by the egg-cell a little more 
closely from this point of view. Every cell takes in a certain amount of potential 
energy in the form of food for its own use. If it leads an active life, either as 
an independent organism or as a constituent part of an organism, it may expend 
by far the greater part, possibly even the whole, of that energy upon its own 
requirements, but usually something is left over to be handed down to its imme- 
diate descendants. If, on the other hand, the cell exhibits very little activity 
and expends very little energy, while placed in an environment in which food 
is abundant, it will tend to accumulate surplus energy in excess of its own needs. 
Such is the case with the egg-cells of the multicellular animals and _ plants. 
Moreover, the oftener the process of absorbing food-material is repeated the 
easier does it become; in fact, the egg-cell establishes a habit of storing up 
reserve material or food-yolk. Inasmuch as it is a blastogenic character, there 
can be no objection to supposing that this habit will be inherited by future 
generations of egg-cells. Indeed we are obliged to assume that this will be the 
case, for we know that the protoplasm of each succeeding generation of egg-cells 
is directly continuous with that of the preceding generation. We thus get at 
any rate a possibility of the progressive accumulation of potential energy in the 
germ-cells of successive generations of multicellular organisms, and of course 
the same argument holds good with regard to successive generations of Protista. 
It would seem that progressive evolution must follow as a necessary result 
of the law of the accumulation of surplus energy in all cases where there is nothing 
to counteract that law, for each generation gets a better start than its prede- 
cessor, and is able to carry on a little further its struggle for existence with the 
environment. It may be said that this argument proves too much, that if it 
1914. oo 
