Creation of a system of classification of living things

Scientists during the 18th century believed, based on this Aristotelian concept, that the Earth was populated with a set of organisms, as distinct species, which had always existed, since their creation, in the same form.

They felt that if they could catalog all of the forms of life, and understand the relationships among them, that they would understand the meaning of life. Species were termed "natural species", with the understanding that they were unchangeable units of living things.

The most successful of the 18th century systematists was the Swede Carl von Linné, also known as Linnaeus. From detailed observance of living things he created a system of classification which related each living thing to all others. It is important to remember that Linnaeus understood that the relation among these species was based on the Great Chain of Being, and not on any relation of descent between the species (which is an evolutionary idea).

Evidence that species were not unchanging

During the 18th century, however, naturalists began to find evidence of relations among species which defied explanation under the prevailing theory.

Biogeographers studied the way that species were distributed on the Earth. They found odd instances of very similar animals separated across seemingly impenetrable barriers. They also found unique species in abundance in places of isolation, like islands (note the bizarre fauna of Australia and New Zealand). They couldn't explain how these came to be.

Comparative Anatomists compared the structures of various species. They found unexpected similarities in structure which had no purpose, such as the pelvic bones in snakes, or the similarity of structure of human hands, bat wings, and whale flippers. If the similarity in organization of these structures could not be explained by the prevailing theory then what would explain them?

Finally geologists began to find evidence of a progression in complexity of organisms across layers of sedimentary rocks. Toward the bottom of such a bed were fossils of simple organisms, while layers above them contained those of more complex organisms, with the fossils at the highest levels most resembling existing organisms. Were these sequences of fossils, that is, did the more complex organisms derive from the less complex?

Gradually, a few scientists started to consider an alternative explanation:

If species did not seem to originate in a single act of creation, and disperse throughout the world, then perhaps they originated in more than one place and perhaps species could be modified over time, as suggested by the comparative anatomy and the fossil record. These evolutionary ideas were prevalent well before Darwin's time.