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Reptiles also are grouped, like the mammals and birds, as variations about a central theme. An ordinary lizard is perhaps the nearest in form to the remote ancestor from which all have sprung. Some lizards are long and very slender, with all four limbs of greatly reduced size. Others, which are still true lizards, have lost the hind limbs, or even all the legs, as in the "blind worms" of England.

They are placed by the mother in a marsupial pouch on her ventral surface, and here nourished till able to care for themselves. Pardon a moment's digression. The marsupials, except the opossum, are confined to Australia, and the oviparous mammals, or monotremes, to New Zealand.

Never having known the species whence our dogs came in its wild state, we are uncertain as to its modes of expression; but, observing the varieties of dogs which are kept by savages, it seems probable that the primitive canines used their voice only in howling or yelping; that is, as a continuous sound akin to the bellowings or other cries of the various wild mammals.

In this respect there is a very conspicuous difference between the lower and higher mammals. In the Marsupials, in which the embryo is only a comparatively short time in the womb and is born in a very immature condition, the vascular arrangements in the yelk-sac and the allantois suffice for its nutrition, as we find them in the Monotremes, birds, and reptiles.

It even becomes occasional, when birds associate against a robber, or mammals combine, under the pressure of exceptional circumstances, to emigrate. In this last case, it becomes a voluntary deviation from habitual moods of life.

She had her redeeming virtues, no doubt; all have, even burglars and murderers, until they slip into the region of pathology; but this morning she looked and felt like a she-wolf; and few mammals are so dangerous, particularly a she-wolf that has never suckled young.

But, in any case, the common descent of man and all the other mammals from one stem-form is beyond question. This long-extinct Promammal was probably evolved from Proreptiles during the Triassic period, and must certainly be regarded as the monotreme and oviparous ancestor of ALL the mammals.

But while birds were more than all other beings to me, mammals too had a great attraction. I have already spoken of rats, opossums, and armadillos; also of the vizcacha, the big burrowing rodent that made his villages all over the plain. One of my early experiences is of the tremendous outcry these animals would make at night when suddenly startled by a very loud noise, as by a clap of thunder.

In the Australian mammals, we see the process of diversification in an early and incomplete stage of development.

These insects are destroyed by birds, small mammals or other insects. The birds are killed by animals and other birds, which in turn are the food of larger animals. And so through all nature runs this law of balance; nothing increases in too great a proportion. But when man comes, he thinks only of his own needs and wishes and begins at once to upset the delicate balance.