Darwin’s Great Tree, Annotated

A cabbage palm near Maungaraho Rock, Northlands, New Zealand

A cabbage palm near Maungaraho Rock, Northlands, New Zealand

➤  In the Origin of The Species by Means of Natural Selection, Darwin wrote eloquently of the Tree of Life, using the academic English language current in 1872. But the language has evolved in the last 140 years.

You can find original text at Archive.org, or as an audiobook from the Librivox project.

In this annotated excerpt of his writing of the Tree of Life, modern phrasing is provided for the concepts he introduces.
1.    The affinities of all the beings of the same class have sometimes been represented by a great tree…     
The taxonomic Tree of Life shows relationships between living things. With the word “class”, he was likely describing the different groupings such as plants, animals, and fungus, which had previously been presumed to have originated independently .
2.    …. The green and budding twigs may represent existing species…        

Living species- Earth’s biodiversity- are analogous to “twigs” and branches on this conceptual Tree.
3.    …and those produced during former years may represent the long succession of extinct species….        

The “extinct species”, which we know from fossils or by anatomical inference, are analogous to the thicker, parent branches, which gave, rise to the current, living species.
4.    …At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have at all times overmastered other species in the great battle for life…

The branches of a tree are growing in competition with one another for light, and this struggle for light is analogous to the survival of the fittest organisms. This competition between branches is a reference to the competition between species for survival, and can be distinguished from the simultaneous struggle to survive against the geographical environment.

A complex tree holds branches in its crown, all connected

A complex tree holds branches in its crown, all connected

5.    The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was young, budding twigs;…  

Here, Darwin is saying that all living things were once related, in a much earlier time. The “limbs” are like the greater classifications of life, such as kingdom (plants vs. animals); the “great branches” are like the subsequent divisions (invertebrates vs. vertebrates); and the “lesser and lesser branches”, even more precise classifications. Notice that he does not mention explicitly that all life stems from one original point.
6.    …and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups…

The “connection of the former and present buds” is a statement that living organisms may be descended from now extinct organisms. All living things have a distinct position on the tree in “groups” within “groups”, or relative to the other branching points within which it is contained.
7.    …Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear the other branches…          

Most lineages of organisms that were alive in the past have died, and only some- perhaps more that “two or three” – have survived and flourished into the present day. The “other branches” are the subsequent organisms that came from those ancient “great branches”.
8.    …so with the species which lived during long-past geological periods, very few have left living and modified descendants…   

Most organisms that were once alive, in the very distant past, were not successful in the long run. Today, all of the new species, the “modified descendants” originate from a small portion of the things that once existed.
9.    …From the first growth of the tree, many a limb and branch has decayed and dropped off; and these fallen branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only in a fossil state….

Since the start of life, many types of organisms have gone extinct and been lost from the Tree of Life (“decayed and dropped off”). These lost species may include vastly different and broad classifications (e.g. birds, pines) or maybe smaller and more precise ones (eagles, crows, sparrows). We know about some of them only because we have discovered their fossilized remains.

A giant kauri tree stretches out its branches

A giant kauri tree stretches out its branches

10.…As we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station…

Some of the more curious members in the biological world, such as the Australian platypus or the South American lungfish, may not seem to fit neatly into a single category. These members may resemble two other “branch” categories because their ancestors branched off from those categories at an early stage. These “branches springing from a fork low down” have seemingly survived in a safe or lucky circumstance.
11.…As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch…
There is an ongoing process whereby new species evolve from living species, and the more successful ones “overtop” and out-survive the weaker “feebler” species.
12.…so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth….
The relationship of organisms can be understood as a “great Tree of Life”, which has grown over vast geological time. The geological layers which record Earth’s history is filled with the remains of extinct organisms, such as dinosaurs, sea scorpions, trilobites, and other groupings which left no direct descendants.
13.…and covers the surface with its ever-branching and beautiful ramifications….
And currently, on the uniquely biodiverse living planet, there are many wonderful species that are continually giving rise to new species yet to appear.