Naturalists are the 19th-century equivalent to biologists in modern-day. Their primary object of study, therefore, was plants and animals. As Darwin characterizes them, naturalists of his period were especially wedded to the notion that the species were created in an unchanging form by God. Since the origins of the species were already explained, then, the naturalist appears to have spent most of time his painstakingly classifying organisms into varieties, species, genera, and higher levels.