He had an extraordinary range & skill of microscopic observations on the microscopic structure of the seeds and embryos of plants & small invertebrate animals. He discovered the existence of spermatozoa & of red blood cells and was thus founder of animal histology. Protozoa, algae, yeasts & bacteria -- all these unicellular organisms were first seen by Leeuwenhoek, often with such accuracy that it is possible to identify individual species from his accounts of them. This microbial world has two distinct features --- diversity and abundance.