36 Gravitation as a Factor [January, 
brain ! But this brain, in order to communicate with the 
external world, requires organs of sensation, organs, too, by 
which it can reaCt upon surrounding matter, and others 
by which it can remove itself from place to place. These 
processes, however, require again the expenditure of energy, 
which the brain has not the power to create out of nothing. 
Hence the necessity for what are sometimes known as the 
vegetative systems of man and animals, — the organs of di- 
gestion, assimilation, circulation, respiration, secretion, and 
excretion, — whose duty is to provide for the waste of tissue 
of the brain and its more immediate accessories, to place at 
their disposal a constant store of available energy, and to 
provide for the removal from the totality thus constructed of 
all matter which has become effete. Such a complex being 
obviously cannot be flung at random into space. It demands 
a world, without which there can be neither scope for its 
activity nor even the possibility of its existence. With this 
world it must be in harmony, and thus the animal becomes 
subject to the dominion of gravitation, with which its organic 
forces aCt now in conjunction and now in opposition. Here 
indeed, just as in the case of heat, we find that there are 
certain limits within which only it is possible for organic life as 
we know it to be maintained. Were the earth’s attractive force 
as exerted upon bodies placed on its surface, either greater 
or less, we should all, — the “ we ” including every animal 
and vegetable organism as well as man, — in virtue of that 
difference alone, be something other than what we now 
are. 
A couple of ideal experiments, the results of which no one 
can doubt, will make this clear, and will at the same time 
explain the part played by gravitation in the animal economy. 
Let us suppose this force doubled. All terrestrial animals 
would then be compelled to exert a vastly increased amount 
of strength in order to support themselves in any other than 
a recumbent position — to rise up from the ground, to run, 
leap, climb, or to drag or carry any objeCt. In order to do all 
this several important changes in the animal structure would 
be necessary. Their muscles must necessarily be more power- 
ful, and the skeleton to which they are attached would need 
a corresponding modification. To work such limbs a more 
rapid transformation of matter would be required ; hence the 
supply of nutriment must be greater, involving enlarged 
digestive organs, and doubtless a larger respiratory apparatus, 
to allow of the perfect aeration of the increased mass of the 
blood. In order to keep up the circulation with the neces- 
sary force, either the heart would require to be more powerful 
