1881.] Geology as a Science and an Art . 451 
fish, but a beast ; and that a bat, though it fly in the air, is 
a beast, and not a bird. For the whale and the bat agree 
with the higher beasts, or Mammalia, in the important cha- 
racteristic of suckling their young. If, therefore, I were 
asked to pick out the two most distinctive features of Science, 
I should take organisation and exactness. 
In further illustration of this, compare our ordinary 
knowledge of the heavenly bodies with that formulated by 
Astronomy. To the ordinary observer the sun shines by 
day, and by night the moon rises and sets, has her periods 
and her phases; Venus or Jupiter is now the first among 
the stars to appear in the evening, and now the last to be 
lost in the more powerful light of day ; Saturn and Mars 
may be seen from time to time ; Orion’s belt, Cassiopeia, 
the Great Bear, and a few other constellations, are well- 
known groups ; for the rest the heavens are more or less a 
maze. To the astronomer, on the other hand, every planet 
is but a unit in a definite system • of that unit the periods of 
revolution round the sun, of rotation on its axis, the figure, 
mass, and path, the motions of its attendant satellites, per- 
haps, and a dozen other faCts are accurately known. The 
sun is not only our source of light, but the ruler of a system 
and the originator of nearly all available forms of energy on 
the earth’s surface : he is not fixed in space, but drifting in 
a known direction and with a calculated velocity : he is not 
merely a ball of fire, but a body of wonderfully complex 
physical constitution and of ascertainable chemical compo- 
sition. Nothing in faCt can better illustrate the gradual 
advance in the organisation and exactness of scientific 
knowledge than the history of astronomy in general, and of 
the solar system in especial. 
In organisation and exactness, then, I repeat, lies the 
essence of Science ; or, to use another metaphor, scientific 
knowledge is crystallised knowledge , as opposed to the com- 
paratively formless or amorphous mass which constitutes 
general information. What, then, is the aim of Science ? 
Clearly to crystallise our knowledge ; clearly to give ordinary 
knowledge that organisation and exactness which it lacks. 
And the aim of the several divisions of science, or sciences 
as we call them, is to develop a consistent body of crystal- 
lised knowledge, which we may call doCtrine, concerning the 
particular groups of natural faCts with which they severally 
deal. 
And now let us see whether the various branches of science 
are all of one kind ; for if they be not, then we shall have 
to note presently to which kind Geology belongs. If we 
