The Lower Animals

Naturalists have long recognized that most invertebrates fall into a small number of basic groups. Aristotle, as we have seen, distinguished various types that he logically grouped together and even observed the behavior and metamorphosis of several insects. Almost everything that was known about them during the middle ages was based on his writings, but there was little understanding of its meaning. The metamorphosis of insects prepared the mind to accept bizarre interpretations that had no basis in fact. When one has seen a butterfly born from a caterpillar, it is not difficult to believe, as Aristotle did, that caterpillars are born from green leaves or that worms are formed in the mud that they inhabit – the same mud from which, according to Genesis, God brought forth man. It was impossible to sort out meaningful ideas and separate them from the complex history of the lower forms of life at a time when there were no objective observations or instruments to augment the power of human senses. It was not until the XVII^th century that the use of magnifying lenses enabled Malpighi, Swammerdam, and Leuwenhoek to study the fine structure of a body and recognize things that, until then were shielded from view by their small size. Malpighi concerned himself mainly with anatomy and embryology and Swammerdam with the metamorphoses of insects. Leuwenhoek used a lens to examine a great variety of objects, and he was the first to point out the existence of Infusoria and to study an animal that budded like a plant, the fresh-water hydra. Trembley’s later research would make it famous. At the same time one of his students at Hamm discovered spermatozoa. The initial impact of all three of these discoveries had a considerable repercussive effect.