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Show 656 PROF. HUXLEY ON THE [Dec. 14, genito-urinary passage and the "genital" region. The first is taken into the bladder and becomes connected by a longer or shorter " cystic urethra " with the latter, which is converted into the longer or shorter vagina. In the Marsupial the same general modification has taken place; but the "genital cornua" become immensely elongated, and give rise to the so-called "double" vagina. Lastly, the marsupium, where it exists, is a no less special feature of the Marsupialia, and, like the peculiarities of the female genital organs, appears to be related with the abnormally early birth of the foetus. Among the higher Mammalia, it is well known that the foetus is born in a relatively much earlier state in some cases than in others, even among closely allied species. Thus Rabbits are b o m hairless and blind, while Hares are born hairy and with their eyes open. I think it probable, from the character of the pes, that the primitive forms, whence the existing Marsupialia have been derived, were arboreal animals ; and it is not difficult, I conceive, to see that, with such habits, it may have been highly advantageous to an animal to get rid of its young from the interior of its body at as early a period of development as possible, aad to supply it with nourishment during the later periods through the lacteal glands, rather than through an imperfect form of placenta. However this may be, the characters of the existing Marsupialia leave no doubt on my mind that they are greatly modified members of the metatherial type ; and I suspect that most, if not all, of the Australian forms are of comparatively late origin. I think it probable that the great majority of the Metatheria, of which I doubt not a great multitude will shortly be discovered in Mesozoic formations, differed widely from our existing Marsupials-not only lacking the pouch, as do some existing "Marsupialia," but possessing undivided vaginae, and probably bringing forth their young not earlier than existing Carnivores and Rodents do, the nutrition of the foetus during prolonged gestation being provided for, in all probability, by an umbilical placental apparatus, and its respiration by a non-placental allantois. In the remaining group of the Mammalia, hitherto spoken of as the "higher Mammalia,"- 1. The mammary glands are provided with teats *-. 2. The cloaca has usually disappeared. Sometimes, however (Beavers, Sloths), a shallow cloaca is present, especially in the female. 3. The openings of the ureters are always eiitocystic; but their position varies greatly, from close to the neck (e. g. Sorex) to the anterior end of the bladder (e. g. Hyrax). 4. There is a distinct vagina, which is almost always undivided. The oviducts are differentiated into uterine and Fallopian portions. 5. The penis is usually large, the bulb single or partially divided, and the corpora cavernosa almost always directly attached to the ischia. 1 The only exception known to me is the Cape Mole (Chrysochloris), which, according to Peters, has none. |