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 

 lias taken place; but the "genital cornua " become immensely 

 elongated, and give rise to the so-called " double " vagina. 



Lastly, the marsupiuni, 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 born hairless and 

 blind, while Hares are born liairy and with their eyes open. I think 

 it probable, from the character of the pes, that the primitive forms, 

 whence the existing IMarsupiaHa 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 peiiod of develop- 

 ment as possible, aad to supply it with nourisliment 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 pro- 

 bable that the great niiijority of the Metatheria, of which I doubt 

 not a great multitude will shortly be discovered in JNIesozoic for- 

 mations, differed widely from our existing Marsupials — not ouly 

 lacking the pouch, as do some existing "Marsupiaha," hut possessing 

 undivided vaginae, and (jrobably 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 proba- 

 biHty, 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 entocystic ; but their 

 position varies greatly, from close to the neck (e. g. Sorex) to the an- 

 terior end of the bladder (e. g. Hyrax). 



4. There is a distinct vagina, which is almost always undivided. 

 The oviducts are diiferentiated 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. 



' The only exception known to me is the Cape Mole (Ckrt/sochlons), which, 

 according to Petera, haB none. 



