Linnaeus, the father of taxonomy, relied on a system of classification based on shared physical characteristics, particularly focusing on morphological traits, which are the observable features of an organism's body. He established a hierarchical system, using these traits to group organisms into increasingly specific categories:

* Kingdom: The broadest category, encompassing all living things (Linnaeus originally had two: plants and animals).

* Phylum: Grouping organisms based on major body plan characteristics.

* Class: Groups organisms with further shared physical traits within the same phylum.

* Order: Grouping organisms with even more specific shared characteristics.

* Family: Grouping organisms with close similarities in morphology.

* Genus: A group of closely related species.

* Species: The most specific level, grouping organisms that can interbreed and produce fertile offspring.

Here are some examples of the traits Linnaeus considered:

* Number of body segments: Insects have segmented bodies, while mammals do not.

* Presence or absence of wings: Birds and insects have wings, mammals do not.

* Type of reproductive system: Mammals give birth to live young, while reptiles lay eggs.

* Type of circulatory system: Birds have a four-chambered heart, while amphibians have a three-chambered heart.

* Type of appendages: Fish have fins, while reptiles have legs.

* Type of covering: Mammals have fur, while birds have feathers.

While Linnaeus's system revolutionized taxonomy, it had its limitations:

* Focus on morphology: It did not account for evolutionary relationships, which were unknown in his time.

* Arbitrary nature: Some traits used for classification were not necessarily indicative of evolutionary relationships.

Modern taxonomy, building upon Linnaeus's work, now uses a combination of morphological, genetic, and evolutionary data to classify organisms more accurately.