We must observe that in no period of sinking after the first land age was the land all covered by water, nor were all the living things of any period destroyed. There has been a continuance of animal life on the globe from the first earth-building time until now. The Permian and Chalk ages were probably the periods of greatest changes. During the Chalk age certain of the later forms of life appeared. The fruit trees, and all the highest and most choice forms of vegetable life, were present among the plants. Fishes and reptiles, crabs and lobsters like those which now live, and the first specimens of the great Order of the Mammals, were found in the troubled seas of the Chalk age. It was with plants that are called exogens or outside growers, having two seed-leaves,[14] such as our fruit trees, and the oaks, maples, chestnuts, and elms, that modern plant life began. When the mammalians came, they were the advance guard of the higher class of animal life. This class of mammalia includes all animals which have young which are nourished with milk by their mothers. The first age of the Neozoic, or New Life period, is marked by a certain singular kind of limestone rock, called nummulite or money-stone. The stone receives this odd name from a great variety of flat shells which fill the rock, and have been fancied to resemble pennies, shillings, and sixpences. These nummulite shells are very curious; some are so small that you can scarcely see them without a microscope; others are as broad as the above-named coins. Each shell is made up of a great number of little cells, set in rings, beginning with a very small ring about the central cell, and the next a little larger, and so on. The shells were the homes of creatures called foraminifera, some of which are now living and building shells in the bottom of our seas. When these creatures were alive these cells were filled with a soft substance such as we see in jelly-fish.[15] We can scarcely realize the vast amount of work done by these small, feeble creatures, in earth-building. The nummulites were the stonemasons of continents. The nummulite sandstone is found in beds, sometimes thousands of feet thick. Imagination fails to compass the multitudes of tiny living creatures which piled up ten thousand feet of the Swiss Alps, and built the snowy peaks of distant Thibet, and grain by grain laid up the long chains of the Pyrenees.