In the quest for demonstrating organic evolution attempts have been made to classify nature into a sequential arrangement, exhibiting a chain of descendents. Easier said than done. Efforts have been pursued by employing the following philosophies (in historic order)-
1) Phenetics - Present nature of organisms - Before evolutionary theories organisms were placed in "buckets" by common attributes, relying on appearance (phenotype). A key developer in our present system was Carolus Linnaeus (1707-1778). In modern times, this method is made more quantitative computer analysis of many features.
2) Phylogenetics - Evolutionary relationships of organisms - This system arose in the late nineteenth century with evolutionary theory. Here grouping are placed by presumed ancestry.
3) Cladistics (from Greek klados branch)- Branching order by which organisms have derived from an ancestral stem. This scheme arose in the 1970's to use a more objective approach based on DNA, proteins etc. Overall similarities are not important.
As we look at different trees, we see that it is difficult to a classify organisms in a consistent way. More elusive is the sequential ordering that should be present if evolution occurred. Evolution would be more feasible if nature's divisions were blurry instead of distinct. Individual species are found at the periphery of trees, but not as ancestors or descendents. We have the leaves, but the trunks and branches are missing.
References:
Frair 1983, 38-40
Denton 1986, 119-141
Milner 1990, 83,84