ence, away from the marshes and the warmth, they developed an outer covering only second in its warmth-holding (or heat-resisting) powers to the down and feathers of the Arctic sea-birds. And so they held out through the age of hardship between the Mesozoic and Cainozoic ages, to which most of the true reptiles succumbed.
All the main characteristics of this flora and sea and land fauna that came to an end with the end of the Mesozoic age were such as were adapted to an equable climate and to shallow and swampy regions. But in the case of their Cainozoic successors, both hair and feathers gave a power of resistance to variable temperatures such as no reptile possessed, and with it they gave a range far greater than any animal had hitherto attained.
The range of life of the Lower Palæozoic Period was confined to warm water.
The range of life of the Upper Palæozoic Period was confined to warm water or to warm swamps and wet ground.
The range of life of the Mesozoic Period as we know it was confined to water and fairly low-lying valley regions under equable conditions.
Meanwhile in each of these periods there were types involuntarily extending the range of life beyond the limits prevailing in that period; and when ages of extreme conditions prevailed, it was these marginal types which survived to inherit the depopulated world.
That perhaps is the most general statement we can make about the story of the geological record; it is a story of widening range. Classes, genera, and species of animals appear and disappear, but the range widens. It widens always. Life has never had so great a range as it has to-day. Life to-day, in the form of man, goes higher in the air than it has ever done before; man's geographical range is from pole to pole, he goes under the water in submarines, he sounds the cold, lifeless darkness of the deepest seas, he burrows into virgin levels of the rocks, and in thought and knowledge he pierces to the centre of the earth and reaches out to the uttermost star. Yet in all the relics of the Mesozoic time we find no certain memorials of his ancestry. His an-