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 turn in Part IV. with no more delay than is required 

 for the citation of the following pertinent extract from Playfair'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 theoi-izing 

 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 instantice crucis, which are 

 the criteria 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). 



