The classification of organisms is an on-going discussion amongst taxonomers. Criterial classification was for quite some time the only method used to classify organisms, organizing them by three categories, diet, body plan, and historical niche. Criterial classification is still used today and still plays an important role in the ideas of conceptual evolution. Conceptual evolution is a recent theory proposed by Jack Horner and built upon by Charles Darwin.
A backbone idea of conceptual evolution is that form is a product of function. When a God created a new organism they would do so by conceptually modifying a preexisting organisms function, thus creating a new form. The more complex organisms functions became the more complex their forms became, this can be observed in simple organisms like algae or moss which serve simple functions and thus have simple forms. Comparatively, an organism like a cow has a very complex function which needs a very complex form to operate.
The idea of modifying existing organisms also gives rise to common ancestors, the idea that any two organisms can have their roots traced back to one common ancestor from which their function derived.