228 



NA TURE 



[Jan. 6, 1 88 1 



iBtermediate position between the Prototheria and the 

 higher mammals. 



1. The mammary glands are provided with teats. 



2. The cloaca is so greatly reduced that it is often said 

 to have disappeared. 



3. The openings of the ureters are entocyslic — that is to 

 say, the ureters open into what is called the " base " of 

 the bladder in front of the narrowed " neck " by which 

 it passes into the tubular ''urethra." This means, I con- 

 ceive, that, morphologically, the bladder of the Marsupial 

 represents the "bladder of the Monotreme + the anterior 

 part of the genito-urinary passage ; the so-called " tri- 

 gonum," if not more, of the bladder of the Marsupial, 

 being the homologue of that anterior segment of the 

 genito-urinary passage of the Monotreme. 



4. There is a distinct and long vagina, quite separated 

 from the cystic urethra, in the female ; and the oviducts 

 are differentiated into uterine and Fallopian portions. 



5. The penis is large, and the corpora cavernosa are 

 connected by fibrous tissue and muscles with the pelvis. 

 The spongy body has a large bifurcated bulb, and 

 Cowper's glands are very largely developed. 



6. The vertebrse have distinct epiphyses. 



7. The malleus is small, and its connections are similar 

 to those which it possesses in the higher mammals. The 

 incus is relatively larger, and the stapes more or less 

 stirrup-shaped. 



8. The coracoid is short, does not articulate with the 

 sternum, and becomes ankylosed with the scapula. 



'9. The hip-gudle is provided with epipubes, usually of 

 large size and well ossified ; and the iliac axis is inclined 

 at a small angle to the sacral a.xis. 



10. The corpus callosum is small. 



ill. In the few forms of which the fcetus is known 

 there is no allantoic placenta ; while the umbilical sac is 

 so large that the possibility of the existence of a transitory 

 umbilical placentation must be taken into account. 



It will be observed that in the characters i, 2, 3, 4, 5, 

 6, 7, 8, and the latter part of the 9th, the Marsupials 

 agree with the higher mammals ; while in the former part 

 of the gth, the 10th, and the nth, they present Proto- 

 therian characters. So far, therefore, they constitute an 

 intermediate type between that of the Prototheria and 

 that of the higher mammals, which may be termed that 

 oi \.\\& Mctatlicria. .And if there were any known animals 

 which combined these characters, with a complete double 

 dentition, unmodified pentadactyle manus and pes, and 

 normal uterogestation, they would furnish us the exact 

 transition between the Prototheria and the higher mam- 

 mals, which must have existed if the law of evolution is 

 trustworthy. 



No known Marsupial, however, possesses these addi- 

 tional characters. None has more than a single succes- 

 sional tooth on each side of each jaw ; and, as Prof. 

 Flower (to whom we owe the highly important demon- 

 stration of this fact) has pointed out, the question arises 

 whether we have here a primary dentition with only one 

 secondary tooth, or a secondary dentition with only one 

 tooth of the primary set left. I have no doubt that the 

 answer given to this question by Prof. Flower is correct, 

 and that it is the milk-dentition of which only a vestige 

 is left in the Marsupialia. Among existing Rodents, in 

 fact, all conditions of the milk-dentition exist, from a 

 number equal to that of the permanent incisors and 

 premolars (as in the Rabbit ') to none at all. 



The same thing is observed in the Insectivora, where 

 the Hedgehog, and probably Centetes, have a full set of 

 milk-teeth, while none have yet been found in the Shrews. 



> T he deciduous molars and the posterior deciduous upper incisurs of the 

 Rabbit have been long kni wn. Eut I have rtcently fund that unbcrn 

 Rabbits p ssess, in addition, two anterior upper and two lower deciduous 

 incisors. B. th are simple conical teeth, the sacs of which are mertly em- 

 bedded in the gum. The upper is net mf re than one-hundreth of an inch 

 long, the lowtr ratVer larger. It wculd be interesting to examine Toetai 

 Guinea-pigs in relation to this point ; at present they are known to possess 

 only the hindmost deciduous molars, so far agreeing with the Marsupials. 



In these cases, it is obvious that the milk-dentition has 

 gradually been suppressed in the more modified forms ; 

 and I think that there can be no reasonable doubt that 

 the existing Marsupials have undergone a like suppres- 

 sion of the deciduous teeth, in the course of their deriva- 

 tion from ancestors which possessed a full set. 



Again, no existing Marsupial possesses an unmodified 

 pentadactyle pes. If the hallux is pre-ent, it presents an 

 extensive movement in adduction and abduction ; in 

 fact, the pes is prehensile. This is the case in the 

 Pliascoloniyida:, Phaldiigistidir, Phasco/a>rtid(r,3.nd Didel- 

 pliidir. The DasyiiiidiT present the same type of pes, 

 with the hallux reduced or suppressed. Hence, consider- 

 ing the relations of the Macropodidce and the Peramelida 

 with the Phalangers, it seems likely that the hind foot 

 in these groups is also a reduced prehensile pes ; in 

 which case this special modification of the foot would 

 characterise the whole of the existing Marsupialia. 



Thirdly, the most marked peculiarities of the re- 

 productive organs and processes in the Marsupial are 

 in no wise transitional, but are singularly specialised 

 characters. The suspension of the scrotum in front of 

 the root of the penis is unlike any arrangement in the 

 higher mammals, and the development of the bulb and 

 of Cowper's glands is in excess of an\ thing observable in 

 them. In the female, the cystic urethra is as completely 

 separated from the vagina as it is in the higher maminals ; 

 while the doubling of the vagina must, in my opinion, also 

 be considered as a special peculiarity which leads from, 

 rather than towards, the higher mammals. In a Mono- 

 treme, in fact, the anterior end of the genito-urinary 

 passage exhibits two very short dilatations or cornua, 

 one on each side. In the middle line, a little distance 

 behind these, the ureters open on a prominent ridge-like 

 papilla. The opening of the bladder lies in front of 

 and below the genital cornua. Now, if we compare this 

 arrangement with that which obtains in the lower forms 

 of the higher Mammalia, we find that the ureteric papillae 

 have separated laterally and moved forwards, in such 

 a manner as to occupy the base of the bladder, and the 

 genital cornua come to lie behind and somewhat dorsad 

 of them. At the same time a longitudinal separation has 

 taken place between what may be called the " ureteric " 

 region of the 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 im- 

 mensely 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 pecu- 

 liarities of the female genital organs, appears to be 

 related with the abnormally early birth of the foetus. 

 Ainong the higher Mammalia, it is well known that the 

 fcetus 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 hairy and with their eyes open. I think it 

 probable, from the character of the pes, that the primiti\-e 

 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 develop- 

 ment as possible, and 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. 1 think it probable that the 



