Page Four 



EVOLUTION 



April, 1928 



The Origin of Mammals 



By Maynard Shipley 



AS is well known, animals below mammals in the 

 evolutionary scale are hatched from eggs in such a 

 condition that they are ready at once to pick up bits of 

 food and thus nourish themselves from the very start. 

 Mammals, on the contrary, must be nourished (suckled) 

 by the mother, even when hatched from eggs laid in a 

 nest, as in the case of the Australian Duck-billed Mole. 



We need not here enumerate the twenty-seven charac- 

 ters which distinguish mammals from all other classes 

 of animals, important as these are to the student of zoo- 

 logy. For the average layman, the important point is 

 that mammals are nourished by means of mammae 

 (breasts), with or without teats, and are, at one stage or 

 another of their lives, provided with hair, though these 

 may later be lost, or more or less obscured. Just as 

 feathers are an indubitable mark of a bird, so hair is an 

 invariable attribute of a mammal. In foetal life, even 

 the glossy-skinned whales are covered with hair, as are 

 human beings; and the baby elephant is born with a 

 covering of hair. The adult whale retains only a few 

 bristle-like hairs on the upper lip. 



Neither the Amphibia nor the Reptilia possess either 

 hairs or feathers, nor glands for secreting milk for their 

 young. Mammals alone possess sweat glands and oi! 

 glands, some being locally specialized, others generalized 

 and scattered over the skin. Reptiles and birds have a 

 dry skin, whereas the Batrachians (Amphibia) have a 

 skin rich in sensitive organs and glands of all sorts. 

 Reptiles and amphibians are cold-blooded, their body 

 temperature changing with the external temperature, 

 usually one or two degrees above the surroundings. 

 Nearly all mammals (and birds) preserve a constant 

 temperature, considerably higher than that of their 

 usual surroundings. The body temperature of mam- 

 mals is capable of varying with safety to the extent of 

 only a few degrees. 



Mammals Derived From Reptiles 



Notwithstanding these and other numerous features 

 in which mammals differ from amphibians and reptiles, 

 if the theory of evolution is valid, the Mammalia must 

 have evolved from either the Amphibia or the Reptilia. 



Years ago Huxley came to the conclusion that the 

 Mammalia were derived from the Amphibia, and this 

 theory was widely accepted. It was believed until quite 

 recently that certain characters in modern amphibia are 

 primitive, and were the same in the early reptiles. This 

 is now known to be a fallacy, thanks to the indefatiga- 

 ble labors of modern palaeontologists. Fossil reptiles 

 found in the Triassic rocks of South Africa show quite 

 clearly that the earliest mammals were derived from 

 some generalized branch of the Reptilia. close to the 

 cynodonts (dog-toothed reptiles). Even in the Permian 

 (just preceding the Age of Reptiles), reptiles with 

 mammal-like features in their skeletons had already ap- 

 peared. In most reptiles the teeth in the front and back 

 portions of the mouth are much alike. In the mammal- 



like reptiles of the Karoo series of strata in South 

 Africa, the teeth are no longer uniform, but are devel- 

 oped into incisors, a canine or "dog-tooth," and a series 

 of premolar and molar teeth behind. It is quite pos- 

 sible that some of these mammal-like reptiles were 

 developing toward a more or less warm-blooded phase, 

 through a change which produced the four-chambered 

 heart and complete separation of the arterial and ven- 

 ous circulation. 



The Monotremata are mammals that lay large 

 reptile-like eggs with a firm leathery shell. While they 

 lay eggs, they are nevertheless mammals, since they 

 nurse their young, somewhat in the manner of the 

 higher mammals, but with an important diiference. 



Milk for Babies 



When the young of these queer creatures are 

 hatched, they are nourished by the mother from primi- 

 tive abdominal milk glands, the milk percolating 

 through a sieve-like place in the skin, buried deep in 

 the hair. The young simply lick off the drops of milk 

 as they drip from the hair. These primitive mammary 

 glands are only modified sweat glands, formed in the 

 skin. Here we meet with a good illustration of the 

 principle that any organ found in a higher animal is 

 invariably derived from a pre-existing organ in a lower 

 form of life. No magic is required — only evolution. 



There are two families of these egg-laying mam- 

 mals, both of which agree in possessing certain marked 

 reptilian characters. When the first specimen was ex- 

 hibited in England, it was at once declared to be a 

 fake, on a par with the manufactured or composite 

 mermaids at that time shown everywhere. One is not 

 surprised to hear of this skepticism. Even seeing is not 

 necessarily believing when one stands before a "furry 

 quadruped with the bill and feet of a duck", an ani- 

 mal with a coat of soft brown fur, that lays eggs, yet 

 gives milk to its young! But such is the miscalled 

 "duck-billed mole" of Southern Australia. 



The other family, represented by Echidna and 

 Proechidna (the latter an inhabitant of New Guinea), 

 is known as the "Australian ant-eater," or "spiny ant- 

 eater." The most common species has a heavy pro- 

 tective covering of stiff quill-like spines like a porcu- 

 pine, with an underlying layer of coarse hair. It has 

 a long snout, no teeth, and a rudimentary tail, much 

 like that of a bird. And it lays eggs like a bird, but 

 only one at a time. It undergoes a short incubation 

 in a marsupial-like brood pouch. The very immature 

 young Echidna lies quietly in the pouch for some time, 

 just able to lick up the milky secretion that exudes ^ 

 from the walls of the pouch, saturating the hairs licked 

 by the young ant-eater. 



The aquatic duck-bill, on the other hand, has no 

 pouch. It lays two or three eggs in a nest made of 

 grasses, similar to a simple bird's nest, the eggs be- 

 ing kept warm by the body heat of the mother. 



