But though the ocean and intertidal waters already swarmed with life, the land above the high-tide line was still, so far as we can guess, a stony wilderness without a trace of life. IV THE INVASION OF THE DRY LAND BY LIFE § 1. _Life and Water._ § 2. _The Earliest Animals._ § 1 Wherever the shore line ran there was life, and that life went on in and by and with water as its home, its medium, and its fundamental necessity. The first jelly-like beginnings of life must have perished whenever they got out of the water, as jelly-fish dry up and perish on our beaches to-day. Drying up was the fatal thing for life in those days, against which at first it had no protection. But in a world of rain-pools and shallow seas and tides, any variation that enabled a living thing to hold out and keep its moisture during hours of low tide of drought met with every encouragement in the circumstances of the time. There must have been a constant risk of stranding. And, on the other hand, life had to keep rather near the shore and beaches in the shallows because it had need of air (dissolved of course in the water) and light. No creature can breathe, no creature can digest its food, without water. We talk of breathing air, but what all living things really do is to breathe oxygen dissolved in water. The air we ourselves breathe must first be dissolved in the moisture in our lungs; and all our food must be liquefied before it can be assimilated. Water-living creatures which are always under water, wave the freely exposed gills by which they breathe in that water, and extract the air dissolved in it. But a creature that is to be exposed for any time out of the water, must have its body and its breathing apparatus protected from drying up. Before the seaweeds could creep up out of the Early Palæozoic seas into the intertidal line of the beach, they had to develop a tougher outer skin to hold their moisture. Before the ancestor of the sea scorpion could survive being left by the tide it had to develop its casing and armour. The trilobites probably developed their tough covering and rolled up into balls, far less as a protection against each other and any other enemies they may have possessed, than as a precaution against drying. And when presently, as we ascend the Palæozoic rocks, the fish appear, first of all the backboned or vertebrated animals, it is evident that a number of them are already adapted by the protection of their gills with gill covers and by a sort of primitive lung swimming-bladder, to face the same risk of temporary stranding.