4!0 DESULTORY SKETCHES IN NATURAL HISTORY. 



some very few cases, as that of the Ornithorhynchus, an idea once 

 prevailed that the essential characters of two classes were combined, 

 it required only more accurate information and increased knowledge 

 of the animal entirely to dispel the illusion, and to destroy altoge- 

 ther the fabulous and mistaken data upon which it was vainly 

 asserted, for a while, by some credulous and superficial writers, to 

 hold a dubious or mediate station.* Descending, however, in the 

 scale, as the groups successively decrease in value, and consequently 

 present less strongly-marked differences, it oftentimes becomes pro- 

 portionally difficult to state their distinctive characters in general 

 terms, to define them with precision and brevity, even though a 

 practised ken may at once recognise them : it being on a combina- 

 tion of many characters that all natural groups are aggregated, the 

 majority of which, but not necessarily the whole, are present in every 

 comprised species ; whence it commonly happens that different of 

 these characters disappear in turn ; so that (even in obvious groups) 

 there may be none of general application. Any one character, 

 therefore, which is peculiar to a group, or in so far peculiar that 

 it does not occur in any proximate division, if applicable to all 

 the members of that group — (which is very frequently the case, as 

 natural groups, however low in the series, are apt to possess such 

 characters) — acquires much >ralue as a means of ready discrimina- 



• It may be proper to remark here that we are quite aware of the import- 

 ant negative relations which the Ovoviviparous Mammalia collectively — viz. 

 the Marsupiata and Monotremata of Cuvier — bear to the Oviparous Vertebrata 

 collectively, as opposed to the ordinary or Placental Mammalia. In the 

 structure of the brain, for instance, the hemispheres of which are connected by 

 a corpus callosum, only in the Placental Mammalia (as recently ascertained by 

 Professor Owen), there being no trace of this in the Ovoviviparous sub-class 

 oi Mammalia, any more than in the three Oviparous classes, while it is almost 

 equally developed throughout the Placental sub-class, in the Beaver propori- 

 tionally as much as in Man. Accordingly, then, there is no gradual linking 

 from one to the other of these two great primary groups of Mammalia in the 

 particular specified, any more than in various other characters unnecessary 

 to detail ; and we are led to recognise, therefore, a dichotomous sub-division of 

 the class, analogous to that of the Reptilia into Ordinary and Batrachian 

 Reptiles, or, in other words, two separate subordinate types of conformation, 

 which do not pass into each other, and the inferior of which is less elevated 

 above the Oviparous classes than the other. In like manner, the Ovovivipa- 

 rous sub-class rigorously sub-divides into the orders Marsupiata and Monotre- 

 mata of Cuvier, the latter of which is again less highly organized than the 

 other, and upon this negative principle is reduced to bear a still closer re- 

 semblance to the Oviparous classes generally, as particularly observable in 

 the simplicity of the construction of the internal ear, &c. 



