16 PRESIDENTIAL ADDRESS, 
give rise to a being with sufficient intelligence to ask how 
and why it was all done. The idea that physical forces 
called into existence indiscriminately, and without any ul- 
terior object, could, by their interaction, evolve the earth 
and all that is on it, is, evidently, quite incredible. But 
this general statement leaves only a vague impression on the 
mind, and, in order to clear our ideas, I will give you two 
examples, one taken from inorganic, the other from organic, 
nature, and treat them in some detail. 
In the first place, let us consider the formation of the 
earth itself. It is evident that no organic development of 
importance can ever take place on the sun; for, when it has 
cooled sufficiently to make the formation of protoplasm pos- 
sible, the temperature of its surface will be rapidly reduced 
to a point below which protoplasm could not live, so that 
there would be no time for life to develop. From this we 
learn that biological evolution can only proceed on a cool 
body, the surface temperature of which is kept nearly 
equable by radiation from another hot body. As these 
conditions must last for a long time, the hot body must be 
large, and at a proper distance from the cool body. But 
much more than this is required for the development of life. 
If living organisms were intended to progress from the ocean 
to the land, in the way I have already mentioned, provision 
must be made for the continuous existence of land from 
the close of the Cambrian period, and this land must be well 
watered. Consequently, the surface of the earth must 
consist partly of land and partly of water, in due propor- 
tion; and the actual amount of water necessary will depend 
upon the size of the earth. The rain, falling on the land, 
constantly washes it down into the sea, and some agency 
must exist for renewing the land by elevation. This eleva- 
tion depends upon the mobility of the crust, which again 
depends upon the internal temperature of the earth. This, 
therefore, it is necessary to conserve. Again, the mass of 
the earth must be sufficiently great to retain on its surface 
by gravitation the water-vapour, which would fly off and 
leave the world dry if the mass was too small. And, once 
more, the materials necessary for supporting life and build- 
ing up organisms must be present. 
From these considerations, it follows that, to secure a 
long development of life, the mass of the earth must be con- 
siderable, and that the cooled crust must be a bad conductor 
of heat. That is, it must be formed of oxides, and not of 
unoxidised metals. There must also be a certain relation- 
ship between the quantities of the several elementary sub- 
stances of which organisms are composed. It is necessary 
