30 JOURNAL, R.A.S. (CEYLON). [VOL. XI. 



still he is unable to separate it from them clearly and 

 satisfactorily, and form it into a new type. 



Only the individual, as Agassiz pointed out, really exists 

 in a material form. Species, genera, orders, classes, and other 

 divisions used by Zoologists are but groupings together of 

 certain series of facts by the human mind,* and the same 

 observation applies to the parts of animals considered sepa- 

 rately. It would be no doubt desirable, if it were possible, 

 in constructing zoological tables to take a definite series 

 of types : for instance, the eagle's wing, the swallow's wing, 

 and the plover's wing, and call them respectively a, 5, and c. 

 Then the student, having become thoroughly acquainted with 

 their respective characters, would, on seeing a, b, c in the 

 wing column, be provided with a mental picture of the kind 

 of wing the species possessed opposite whose name the letter 

 stands. But this aim is too high ; the idea of a type at first 

 sight so precise and clear is really of the most evanescent 

 kind when a long series of animals is studied. Take the 

 case of the head shields in snakes. 



In No. 24, Zamenis mucosus, the head is entirely shielded 

 above. There are a pair of internasals and a pair of f rontals ; 

 the nasals are small and lateral, and there is a mental groove 

 with chin shields. All these characters are of wide generality 

 and easy to determine, being points that make a strong impres- 

 sion on the eye. The two genera Simotes and OUgodon, spec ies 

 20 to 23, agree with Zamenis perfectly in these characters ; 

 but the rostral shield is much larger, and the internasals greatly 

 reduced in size. When first taking up the study of Ophiology 

 the student will not be struck with these differences ; but 

 when he learns how alike are the head shields of all those 

 species that I have arranged under letter a, although belong- 

 ing to such widely different groups, — ground-snakes, tree- 

 snakes, water-snakes, snakes diurnal and nocturnal, poisonous 

 and non-poisonous, — he will not fail to be struck by the strong 



* Introduction to " The Natural History of the United States. 



