66 BIOLOGICAL LECTURES. 



shown, and in so many different fields when applied in cases 

 where verification has been possible, that one may well hesitate 

 about holding very fast to any conclusion which has not yet 

 been adjudicated by experience. This holds as true for what 

 we call philosophy as for science. We are in possession of a 

 body of scientific doctrines or propositions about material 

 phenomena which are satisfactory to this extent : they have 

 met all the criticism to which they have been subject by men 

 of every nation who have concerned themselves with them. 

 Such, for instance, are the doctrines of mathematics, of astron- 

 omy, of geology, and some others in physical science, all of 

 which deal with verifiable matters, that is, with matter and the 

 forms of energy and their relations. In gaining this degree of 

 certainty there has been a series of steps taken tentatively, 

 engrossing the attention of men interested in science for a 

 century or two. The thing to note here is, that obvious as 

 many of these propositions are to us, now they are pointed out, 

 they were so far from obvious to our predecessors that almost 

 every other conjecture was entertained, and often with no 

 attempt at verification before the present ones were adopted. 

 This is of so much importance that some examples may be use- 

 ful to make plain the meaning. Take the case of the explana- 

 tion of the position and motions of the earth, and other bodies. 

 First, as to the center of creation about which sun, moon, and 

 stars revolved. The apparent was taken to be the truth, and 

 the difficulties of such an explanation of the apparent were 

 quite ignored, and the necessity of having some sort of an 

 answer to the question as to the causes of day and night was 

 so great that such an explanation was thought to be better 

 than nothing. After that came Ptolemy's explanation, a won- 

 derful and intricate system of cycles and epicycles which 

 mechanically would not work, but served for a time better than 

 the one it displaced. Then came Copernicus with the plan of 

 the sun as the center with the planets and the earth revolving 

 about it. This simplified the problem, but the reason given 

 for supposing the orbits of the earth and the planets to be 

 circular was not a physical reason, that is, not an observed 

 or calculated one, but a metaphysical reason ; that is, one 



