Remarks on Classification of Vertebrata, ; 915 
General Features of the Scheme. Dichotomy.—The twofold . 
_ division of groups is not new in practice, and is, as it seems to 
_ the writer, becoming more common,—e.g., the primary division 
of animals into Protozoa and Metazoa in place of four or more 
= “branches”; the primary division of vertebrates into Acrania 
_ and Craniota, and the combination of Reptiles and Birds as Sau- 
_ fopsida, But, apparently, there has not been hitherto a distinct 
- fecognition of dichotomy as a fundamental principle in natural 
_ Glassification, or an expression of doubt as to whether any group 
kally comprises three or more equal and co-ordinate subdi- 
Visions, The writer’s view may be briefly stated as follows: 
 dnany assemblage of three or more objects, individuals, or groups, 
‘ two or more of these units are more nearly related to one another 
than to the third or the others, indicating a primary division of the 
Mire assemblage, Very commonly the basis for the distinction 
WS the presence or absence of an organ, feature, or condition, 
Whence arise positive and negative names, like vertebrate and in- 
Vertebrate, amniota and anamntota, etc. 
: Itrespective of special facts, then, the writer has been led to 
 Coubt the naturalness of, for example, the popular subdivision of 
Maral objects into three co-ordinate kingdoms, animal, vegetable, 
À ind mineral; of the chordata into wrochorda, cephalochorda, and 
enka > Of vertebrates into Ichthyopsida, Sauropsida, and 
ammatia, and of mammals into Prototheria, Metatheria, and 
- shew Polychotomy is probably never more than provisional, 
and all classification will eventually be dichotomous. 
7 e ra and Taxonomic Importance of the Central 
Ystem—The superior taxonomic value of the brain and 
pe Da insisted upon by the writer in a paper read before this 
from Ba in 1875 ; during the last seven years, as may be seen 
ous publications, he has become more and more im- 
i with the profound morphological significance of the 
wa and modifications of the cavity of the central, nervous 
on A y the neurocæle, 
EA cele persists in all vertebrates, including Branchi- 
val. and is present in the early stages of all Tunicates whose 
“eae is known. So far as the writer has been able ig. 
“a, the central nervous system is neither tubular nor even 
Ricates Mt any stage with any “ Invertebrate,’ excepting the Tu- 
3 een, perhaps, Balanoglossus. 
