326 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
involved repeated alternations of external and internal work; the col- 
lection of observations and the induction of generalizations on the one 
hand ; and on the other hand the invention of hypotheses, the deduc- 
tion of their consequences, the confrontation of deductions with general- 
izations, the evaluation of agreements, and the repeated revision of the 
whole process. It is not profitable to expose the personal history of a 
study all through these stages, for the convenience of the reader is best 
served by a careful separation of its two phases; and to the second of 
these we may now turnin Part IV. with no more delay than is required 
for the citation of the following pertinent extract from Piayfair’s Illustra- 
tions of the Huttonian Theory of the Earth. After pointing out that to 
wait for the completion of discoveries in other sciences before theorizing 
in geology “would not be caution, but timidity, and an excess of pru- 
dence fatal to all philosophical inquiry,” this lucid writer of a century 
ago proceeds as follows : — 
“The truth, indeed, is, that in physical inquiries, the work of theory 
and observation must go hand in hand, and ought to be carried on at the 
same time, more especially if the matter is very complicated, for there 
the clue of theory is necessary to direct the observer. Though a man 
may begin to observe without any hypothesis, he cannot continue long 
without seeing some general conclusion arise; and to this nascent 
theory it is his business to attend, because, by seeking either to verify 
or to disprove it, he is led to new experiments, or new observations. 
He is also led to the very experiments and observations that are of 
the greatest importance, namely, to those instantiee erucis, which are 
the eriteria that naturally present themselves for the trial of every 
hypothesis. He is conducted to the places where the transitions of 
nature are most perceptible, and where the absence of former, or the 
presence of new circumstances, excludes the action of imaginary causes. 
By this correction of his first opinion, a new approximation is made 
to the truth; and by the repetition of the same process, certainty is 
finally obtained. Thus theory and observation mutually assist one 
another; and the spirit of system, against which there are so many and 
such just complaints, appears, nevertheless, as the animating principle 
of inductive investigation. The business of sound philosophy is not to 
extinguish this spirit, but to restrain and direct its efforts ” (524, 525). 
