Summer is ending, and I’ve migrated back to Copenhagen for the rest of the year.

Being an ecologist, one of the first places I went to visit was the botanical garden and the university’s geological museum. I stumbled upon an exhibit of the Flora Danica – a guide to all the plants of Denmark, published under royal authority in 1761. The flora consists of thousands of engravings, each drawn carefully to help the reader identify the species from flowers, leaves, stems, and root. Color prints made from the engravings were distributed freely to bishops around the country, with other editions’ prices subsidized by the crown. It is a beautiful piece of work, but what struck me most was its complete lack of organization. Today, we have a robust system for organizing species, which we depend on heavily for a range of science and conservation applications. To me, seeing a book of plants without an organization scheme is like imaging a dictionary that isn’t published in alphabetical order. To understand why, we have to go back in time.

In the 1700s, taxonomy (the study of classification of organisms) was a new subject. Linnaeus published his famous Systema Naturae in 1735, but his ideas were by no means well-accepted immediately, nor was his approach based on arguments deeper than morphological similarity. As a result the Flora Danica, published a few decades later, was produced as an arbitrarily ordered set of color plates. Each publisher or book-binder could arrange these plates however they pleased. While charming, that system made it difficult to generalize and think more scientifically about plants. In fact, it was unclear that there should even be a correct way to organize species (why not group by poisonous and not-poisonous, for example) until the advent of evolutionary biology. Darwin showed that species arise from other species, providing a natural organization – more related species in the same groups, because they share the same evolutionary history. Gnetum, for example, looks passingly similar to Apocynum, but the groups are actually separated by tens of millions of years of evolution.

Modern cladistics, under Hennig, pushed the evolutionary idea forward and gave us the basis for our modern groupings of species.Below, you can see a small part of the botanist Alwyn Gentry’s monumental work, A Field Guide to the Families and Genera of Woody Plants of North west South America, giving the characters that help to separate different evolutionary groups of plants.

Illustrations have also changed – the lush detail of the Flora Danica is replaced by sparse line drawings, equally informative, but perhaps missing some indefinable romanticism of the earlier work.

Modern cladistics and molecular biology work – DNA sequencing, for example – have almost made it possible to know the full plant tree of life. You can see an evolutionary tree from one of the more recent studies (from 2010) below.

I think that in the next few decades the work of classifying all the plants of the world – and perhaps, all the known species – will be largely complete. We are riding a wave of advances in computer power and DNA sequencing methods that will build us a complete tree of life. Such a thing – a catalog and moreover an index to all the species that live – would surely be inconceivable to the royal botanists compiling a flora all those centuries ago.