and its Significance in Systematie Zoology. 83 
Every one knows, whether he have thought about it or other- 
wise, that the four Vertebrate classes are homomorphically con- 
nected. Thus there are Flying Mammals, such as the Bats 
and Flying Squirrels (Péeromys), uniting them with the Class 
Aves; as well as that anomalous Monotreme, the Ornithorhyn- 
chus, or Web-footed Duck-bill. The Edentata among Quadru- 
peds connect them with Reptiles, by means of the Armadillos,— 
the Great Armadillo (Dasypus gigas), and preeminently the 
Mataco (D. Apar), being homomorphic of the Testudinata, while 
to the Saurian Reptiles they are united by the Scaly Pangolins 
(Manis), and to the extinct Pterosaurians (Pterodactyles), again, 
they are united by the Bats. With Fishes, the Mammalia are 
most singularly connected by the Cetacea; while a special re- 
semblance appears between the Narwhal (Monodon) and the 
Swordfish (Xiphias). 
The homomorphic resemblances between Birds and Reptiles 
are not striking; but the Draconine Saurians or Flyig Lizards 
(Draconis, sp.) supply examples, and the extinct Pterodactyl 
once afforded another; while with Fishes the various species of 
Flying-fish (Ezocetus) among the soft-finned, and Flying Gur- 
nards (Dactylopterus and Pterois) among the hard-finned, are 
good illustrations. It only remains to connect Reptilian forms 
with Fishes; and here the Snakes (Ophidia) may well be com- 
pared with the Eels; and less striking instances of resemblance 
occur between the Saurian reptiles, such as the Alligator, and 
the bony-cased Sturgeon, and between the Testudinata and the 
Trunk Fishes (Ostracion). Perhaps also that great Enaliosaur 
the Ichthyosaurus might be here mentioned. 
Without extending my illustrations too far, I will select the 
Mammalia as an example of the recurrence of form within the 
limits of a single Class. The organic structure and affinities 
of one Order are dissimilar from those of another, just as the 
structure and affinities of one Class differ from those of another ; 
the difference between Class and Order being one of degree, and 
not of kind; so that it is as remarkable to find resemblances of 
form in widely separated Orders as in still more widely sepa- 
rated Classes, although, of course, homomorphic resemblances 
are more striking between Orders than between Classes. In the 
Order Quadrumana, for instance, we shall find representative forms 
of various other Orders. Thus the genera Midas and Jacchus, 
known as Marmozets, true Platyrrhine Quadrumana, represent 
the Rodentia through the genus Sciurus (Squirrel) ; and the 
Douroucouli (Nyctipithecus felinus), in the same division, repre- 
sents the Cat (Felis) in the Digitigrade Carnivora ; while, among 
the Strepsirrhine Quadrumana, the Loris (Stenops tardigradus) 
represents the true Sloths in the Order Bruta, -_ the very 
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