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page 3 K? Tals materlal^on^of the most durable building materials used by liv¬ ing forms, fungus spores having been found hundreds of million years old, with the chltln envelope still unchanged* everyone is familiar with the ease of preservation of insect specimens in the museum, a further/ indication of the durability of this material* Chitin may be formed from nitrogenous materials and carbohydrates and slnce^nese materials are esentlal to life and are abundantly distributed in the world about us, there would neve^have been a shortage of supply had our primeval ancestor elected to use tills as a structural material for the skeleton* Silica, the material of sand, which is so abundant in the world is used as skeletal material by many microscopic forms, and is as permenant as the rocks, but /// it is only slightly soluble, uiid was not available in sufficient amount to tempt our eretaeean ancestor to us it as a skeletal building materal* Calcium and magnesium cx phosphate or carbonte have been used as the structural unit by many forms of animals, flan and shellfish, as well as by microscopic / organisms* Of ti^/esf compounds, calcium phosphate is the most insajU*'^^* and therefore the most permanent skeletal material, and was developed as a skeleton by o«r evolutionary ancestor* Calcium was more abundant in the days of our evolutionary ancestor than it is now# In the 400,000,000 years that have passed, ealqjf^ri has become scarce C Billions of tons/ of lime have used to form skeletons, which, when the animals died, have only very slowly weathered and gotten back Into the calcium cycle of nature* JSnormous deposits of limestone, chalk and phosphate rock, are formed entirely of tue skeletons of marine animals* This gr^at amount of calcium is largely withdrawn from circulation, and only slowly finds i/ts wa$ back into solution, and into taose plant and animal foods that are the necessary sources of t.J.s vital food ele¬ ment for man.