One of the first tasks the biologists of the C19th faced was grouping the plants and animals they saw around them. Once a group was recognised it was than possible to discuss variation within that group and the relationship between groups.
One of the scientists at the forefront of classification was Carl Linnaeus. He devised a system of classification which started with a simple division of everything into Kingdoms, these were than subdivided until the smallest unit was reached the species. His system is the one which was eventually adopted worldwide.