f.] EDUCATIONAL VALUE OF NATURAL HISTORY SCIENCES. 79 



by Physiologists themselves — that Biology differs from 

 the Physico-chemical and Mathematical sciences in 

 being " inexact." 



Now, this phrase "inexact" must refer either to the 

 methods or to the results of Physiological science. 



It cannot be correct to apply it to the methods ; for, 

 as I hope to show yon by and by, these arc iden- 

 tical 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 respiration 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 vertebratcd animal never open sideways, but 

 always up and down ; while those of an annulose animal 

 always open sideways, and never up and down — I am 

 enumerating propositions which are as exact as anything 

 in Euclid. Plow then has this notion of the inexactness 

 of Biological science come about? I believe from two 

 causes : first, because, in consequence of the great com- 

 plexity of the science and the multitude of interfering 

 conditions, we are very often only enabled to predict 

 approximately what will occur under given circum- 

 stances ; and secondly, because, on account of the com- 

 parative youth of the Physiological sciences, a great 

 many of their laws are still imperfectly worked out. 

 1 kit, in an educational point of view, it is most important 

 to distinguish between the essence of a science and 

 the accidents which surround it ; and essentially, the 

 methods and results of Physiology arc as exact as those 

 of Physics or Mathematics. 



It is said that the Physiological method is especially 

 comiKireitive l ; and this dictum also finds favour in the 



1 •' In the third place, wc have to review the method of Comparison, which 

 is so specially adapted to the study of living bodies, and by which, above all 



