222 STUDY OF NATURAL HISTORY. 



while we see and admit it in her grander features : 

 besides, it is not to be supposed that such forms as 

 we have elsewhere cited (150.), are scattered indis- 

 criminately in their respective groups, without being 

 accompanied by others, equally representing each 

 other, and therefore implying, in the strongest 

 possible manner, the existence of strict uniformity. 

 We may, then, safely conclude, that if the number 

 of our genera in a sub-family disagrees with the 

 number of divisions in our genera, the fault lies 

 with ourselves. We must again retrace our steps, 

 perhaps abandon altogether the number first assumed 

 as definite, and adopt some other more in unison 

 with the facts before us. If, on the contrary, we 

 can, in these new and higher groups, demonstrate 

 the same prevalence of a determinate number, the 

 strength of our theory is doubled. It has been well 

 observed*, that, " whatever error we commit in a 

 single determination, it is highly improbable we 

 should always err in the same way ; so that, when we 

 come to take an average of a great number of de- 

 terminations (unless there be some constant cause 

 which gives a bias one way or the other), we cannot 

 fail, at length, to attain a very near approximation 

 to truth ; and, even allowing a bias, to come much 

 nearer to it than can fairly be expected from any 

 single observation, liable to be influenced by the 

 same bias." This useful and valuable property of 

 the average of a great many observations — that it 

 brings us nearer to the truth, — that is, to the deter- 

 mination of a prevalent number, — than any single 



* Hersch. Discourse, p. 215. 



