454 THE CAUSES OF THE X I 



physiological characteristics of species, and I not 

 only think that they are competent to account for 

 them, but I think that they account for many 

 things which otherwise remain wholly unaccount- 

 able and inexplicable, and I may say incompre- 

 hensible. For a full exposition of the grounds on 

 which this conviction is based, I must refer you to 

 Mr. Darwin's work ; all that I can do now is to 

 illustrate what I have said by two or three cases 

 taken almost at random. 



I drew your attention, on a previous evening, to 

 the facts which are embodied in our systems of 

 Classification, which are the results of the examin- 

 ation and comparison of the different members 

 of the animal kingdom one with another. I men- 

 tioned that the Avhole of the animal kingdom is 

 divisible into five sub-kingdoms ; that each of these 

 sub-kingdoms is again divisible into provinces; 

 that each province may be divided into classes, 

 and the classes into the successively smaller groups, 

 orders, families, genera, and species. 



Now, in each of these groups the resemblance 

 in structure among the members of the group is 

 closer in proportion as the group is smaller. Thus, 

 a man and a worm are members of the animal 

 kingdom in virtue of certain apparently slight 

 though really fundamental resemblances which 

 they present. But a man and a fish are members of 

 the same sub-kingdom Vertcbrata, because they are 

 much more like one another than either of them. 



