The basic principles of biological classification are universal, but it is traditional for a botanical student to approach the subject in the first instance through an examination of the flowering plants. Because the angiasperms are the only major group of plants in which the classification can be understood without the use of the microscope and without any knowledge of fossil forms, they were the first group of plats to be effectively classified and the factors which made them specially easy for the early botanists still operate to the advantage of the modern student. Land plants first appear with any frequency in Silurian rocks but recent discoveries of land plants in Cambrian rocks indicate a much earlier origin. To be useful in combating disease in animals an antibiotic must show no serious toxicity to animal tissue. Many antibiotics are valueless for therapeutic use since not only are they toxic to pathogenic micro organisms but also to higher animals.
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