In the first place, there is the fascinating story of the origin of vertebrates from invertebrates of the sea scorpion or spider type.

Geologically this is a very ancient region, having appeared above the waters before the dawning of the age of reptiles, or, indeed, of any true land vertebrates on the globe.

Between these two extremes we have all degrees, from the innumerable cells that build up the body of the highest Vertebrate to the single-celled Worm, and from the myriad cells of the Oak to the single-celled Alga.

The Mammalia are also warm-blooded and breathe through lungs; but they differ from all other Vertebrates in their mode of reproduction, bringing forth living young which they nurse with milk.