664 MAMMALS 



(A) MONOTREMES AND MARSUPIALS 



In these 'lower' mammals the eye alone would prove the reptilian 

 origin of the whole mammalian class. Indeed, with the exception of 

 exactly two features — one of them outside the eyeball (in the oculo- 

 rotatory musculature) and the other one inside (in the ciliary body) — 

 the monotreme eye is so completely reptilian that it affords no am- 

 munition for use against those few mammalogists who claim separate 

 reptilian origins for the monotremes and for all other mammals. 



The marsupials originated as opossum-like animals, and only such 

 forms (together with Ccenolestes) have been able to survive in the 

 American home of the group. In Australia however, where they became 

 isolated from placental flesh-eaters, the marsupials differentiated into 

 a number of types, many of them imitative of placental types. Thus, 

 there are marsupial mice, rats, marmots, rabbits, flying-squirrels,* jer- 

 boas, bears, cats, wolves, ant-eaters, and golden moles. There was once 

 even a marsupial 'lion', though it was probably a mild-mannered vege- 

 tarian. The marsupials have avoided the water, so there are no marsupial 

 seals or porpoises — the tropical American water-opossum, Chironectes, 

 is the only aquatic marsupial. Nor have the marsupials developed any 

 hoofed types; but the larger kangaroos fill about the same ecological 

 niche. 



The lower marsupials are mostly carnivorous and the higher types 

 (phalangers, kangaroos) herbivorous. Most marsupials, like the mono- 

 tremes, are crepuscular or nocturnal to some degree; but the larger 

 kangaroos are arhythmic and a few are quite strongly diurnal. In keep- 

 ing with the adaptive radiation of the marsupials, their eyes show great 

 differences from form to form. In proportion to the number of species, 

 they have had woefully little attention as compared with the placentals. 

 The marsupials are really the central group of mammals, and deserve 

 much more thorough exploration, from all biological viewpoints, than 

 they have ever yet received. 



The Monotreme Eye — The eye of Ornithorhynchus has been de- 

 scribed only once, by Gunn in 1884 from material preserved in whisky 

 by a Mr. Sinclair, who clearly took his science very seriously. The eyes 

 of the two genera of echidnas have been described by Franz, by Kolmer, 



*A fascinating coincidence is that the flying-squirrel type has been evolved more than once 

 in the rodents — by the true flying-squirrels and in the Anomalurid»- — and more than once 

 also in the marsupials: there are three kinds of flying phalangers, each a close relative of a 

 different non-flying form. 



