II OF THE NATURAL HISTORY SCIENCES 47 



ly admitted by Physiologists themselves — that Bi- 

 ology differs from the Physico-chemical and Math- 

 ematical sciences in being " inexact." 



Now, this phrase " inexact " must refer either 

 to the methods or to the results of Phvsiolooical 

 science. 



It cannot be correct to apply it to the methods; 

 for, as I hope to show you by and by, these are 

 identical in all sciences, and whatever is true of 

 Physiological method is true of Physical and 

 Mathematical method. 



Is it then the results of Biological science which 

 are " inexact " ? I think not. If I say that respi- 

 ration is performed by the lungs; that digestion is 

 effected in the stomach; that the eye is the organ 

 of sight; that the jaws of a vertebrated animal 

 never open sideways, but always up and down; 

 Avliile those of an annulose animal always open 

 sideways, and never up and down — I am enumer- 

 ating propositions which are as exact as anything 

 in Euclid. IIow then has this notion of the in- 

 exactness of Biological science come about? I 

 believe from two causes: first, because in conse- 

 quence of the great complexity of the science and 

 the multitude of interfering conditions, we are 

 very often only enabled to predict approximately 

 what will occur under given circumstances; and 

 secondly, because, on account of the comparative 

 youth of the Physiological sciences, a great many 

 of their laws are still imperfectly worked out. 



