Why did the approach to the study and classification of animal species change during the nineteenth century

Why did the approach to the study and classification of animal species change during the nineteenth century? What made zoologist or biologist approach these studies and classification differently?

Darwin's theory of natural selection as the reason new species arise from existing species means that all living things on earth can be traced to a common ancestor. Besides, even before his proposal, scientists were already realizing that living species are related to one another through common ancestry. Therefore it is no longer tenable just to group together species that are alike, e.g. Linnaeus's Vermes (worms), but it is necessary to figure out whether worms actually share a more recent common ancestor with one another or whether they merely resemble each other because of similar ways of life, since form follows function. Indeed, within the pages of Darwin's Origin of Species, Darwin was discussing how he and his contemporaries were relying on vestigial characters rather than adaptive ones to ascertain relationships, because functional features tend to be the products of convergent evolution, whereas vestigial (or selectively neutral) features are more likely to have been inherited from a common ancestor.

Therefore, ever since Darwin, classifications must take into account both genealogical relationships and morphological similarities. Superficially similar organisms that do not share a recent common ancestor will no longer be recognized (knowingly anyway because the bird-theropod nexus is a currently recognized taxon that is polyphyletic) as a valid taxon , because such groups are considered polyphyletic. Only monophyletic (a term coined by Haeckel) groups will be recognized.