which live in the ocean but lay their eggs in rivers, and these are able to endure the change from the one kind of water to the other without being destroyed. They, to be sure, make the transition gradually, swimming up from the ocean into water that is less and less salty, until they finally reach the fresh water stream itself. There are other kinds of living things which can endure a much more abrupt change from salt to fresh water and back again. In various parts of the world are large rivers emptying into the ocean and so situated relative to towns and cities that steamers make regular round trips from the ocean up to the fresh water of the river and back again, and it will be found that on the bottoms of these steamers are various plants and water animals which endure the frequent shift from salt water to fresh and back again without harm. The percentage of salt in the body fluids of all the higher animals including ourselves is only about one-fifth that in the ocean. Furthermore the fluids of most kinds of land animals have about the same percentage of salts dissolved in them. Naturally there has been much speculation as to why there should be this percentage of salt in preference to any other. One ingenious theory supposes that back in the beginning of things, when the earth cooled down below the boiling point of water, so that it was possible for water to collect on the earth, the water in all the oceans was fresh. This would have to be true, since water must have fallen in the form of rain; but in course of time some of the salts in the earth’s crust would be dissolved, making the water salty, and as time went on the ocean would become saltier and saltier. This is still happening, for every river that discharges into the ocean carries with it materials that it has dissolved from the underlying soil during its passage from its source, and such material, when it once enters the ocean, must stay there until the ocean water becomes saturated with that particular substance. According to this theory there was a time, then, when the ocean water was just about as salty as our body fluids at the present time, and it supposes also that that was the time when the ancestors of present land animals crawled out of the ocean and took up their abode on land. Of course there is no way to prove that this is so, but it does account for the particular percentage of salts in our bodies as well as any other explanation we know anything about.