18 
MONOTREMATA. 
Mammalia presents species of comparatively high and others of lower grade of 
organization, and I think it very certain, then should we be prepared to find, 
in the lowest order of the class, a great amount of difference, as compared 
with the higher orders, in those characters which, as they approximate to, or 
deviate from, a certain standard, are said to indicate a higher or lower gradein 
the scale of organization. 
Now it is precisely in such characters that the most important distinctions 
of the section Marsupial a are manifested ; and the raising that group to a rank 
above an order, is to admit that the amount of difference is greater than could 
be, d priori, anticipated in the lowest order of a class; and yet this most 
important branch of investigation has not, to my knowledge, been considered. It 
remains to be inquired whether there is not an increase in the ratio of the amount 
of differential characters as we descend in the scale, and whether there may not 
be an increase in the amount of variation exhibited in the species of the lowest 
division of any great group. There arc not only grounds for believing such 
to be actually the case, but I think the embryologist would be prepared to 
account for some of these points—partly, perhaps, by the more rapid changes 
in the metamorphoses which a high animal undergoes in the earlier stages of its 
existence. 
In the foregoing pages I have not alluded to the Monotremata , for although 
it is generally admitted that that group possesses a relationship to the J hr- 
supiata, the nature of that relationship can only be determined by such 
investigations as I have above alluded to. If the views which are hinted at 
in this note 1 be well grounded, then is the Monotremata a family of the order 
Marsupiata. 
SECTION I.—MONOTREMATA 2 . 
Mammalia possessing marsupial bones, wanting the corpus 
callosum to the brain, with the mass called corpora quadrigeminu 
divided by a transverse fissure, and with the posterior part undi¬ 
vided : the sternum and shoulder bone3 joining, and encircling 
the fore part of the trunk: the sternum with the manubrium 
joined in front by an episternum, which is produced on each 
side at its anterior extremity into a long branch, attached to and 
1 I sec no reason why similar considerations should not be brought to our 
aid with a view to determine the rank of the section Amphibia among 
Reptiles, about which there has been so much discussion. 
- From p6i‘os, unieus; rpijpa , foramen. 
