65:6.4

One of the most serviceable and complex episodes in the evolution of the higher types of animals consisted in the development of the ability of the iron in the circulating blood cells to perform in the double role of oxygen carrier and carbon dioxide remover. And this performance of the red blood cells illustrates how evolving organisms are able to adapt their functions to varying or changing environment. The higher animals, including man, oxygenate their tissues by the action of the iron of the red blood cells, which carries oxygen to the living cells and just as efficiently removes the carbon dioxide. But other metals can be made to serve the same purpose. The cuttlefish employs copper for this function, and the sea squirt utilizes vanadium.