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Updated: June 24, 2025
There is no question here of higher psychic action. If we compare these sluggish parasites with the intelligent and active ants, we must admit that the psychic differences between them are much greater than the psychic differences between the lowest and highest mammals, between the Monotremes, Marsupials, and armadillos on the one hand, and the dog, ape, or man on the other.
All these remarkable reptilian characters must have been possessed by the stem-form of the whole mammal class, the Promammal of the Triassic period, and have been inherited from the Proreptiles. During the Triassic and Jurassic periods the sub-class of the Monotremes was represented by a number of different stem-mammals.
In them the glands have no teats for the young animal to suck, but there is a special part of the breast pierced with holes like a sieve, from which the milk issues, and the young Monotreme must lick it off. Further, the brain of the Monotremes is very little advanced. It is feebler than that of any of the other Mammals.
This organ is only found in the mammalian class. It is very rudimentary in the lowest section, the Monotremes. In the others it is found at very different stages of development, and sometimes of degeneration. It is degenerate in most of the aquatic mammals. The majority of them have lost it altogether for instance, the walruses and whales and most of the seals.
In the same way their affinities are seen to be with the lowest of mammals, the MONOTREMES and MARSUPIALS. The monotremes, such as the duckbill mole and the spiny ant-eater of Australia, reproduce by means of eggs resembling those of reptiles; the marsupials, such as the opossum and the kangaroo, bring forth their young alive, but in a very immature condition, and carry them for some time after birth in the marsupium, a pouch on the ventral side of the body.
As a matter of fact, the Theromorpha are nearer to the Mammals in the chief points of structure than any other reptiles. The whole structure of their pelvis and hind-feet has attained the same form as in the Monotremes, the lowest Mammals. The formation of the scapula and the quadrate bone shows an approach to the Mammals such as we find in no other group of reptiles.
This second sub-class of the Mammals is very interesting as a perfect intermediate stage between the other two. While the Marsupials retain a great part of the characteristics of the Monotremes, they have also acquired some of the chief features of the Placentals.
It might just as well have been like that of some other marsupial; or even like that of the totally different group of Monotremes, of which the only living representatives are the Echidna and the Ornithorhynchus.
The bladder also opens into the cloaca in the Monotremes, and, indeed, apart from the two urinary ducts; in all the other Mammals the latter open directly into the bladder. It was proved by Haacke and Caldwell in 1884 that the Monotremes lay large eggs like the reptiles, while all the other Mammals are viviparous.
This would seem to indicate that the milk gland was not a hypertrophied sebaceous gland, but a distinct outgrowth, which however had nothing to do with sweat glands. That the intra-uterine gestation, or its cessation, were not originally necessary to determine the functional periodicity of the milk glands is proved by their presence in the Monotremes, which are oviparous.
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