File:Darwin tree.png

English: Charles Darwin's 1837 sketch, his first diagram of an evolutionary tree from his First Notebook on Transmutation of Species (1837) on view at the Museum of Natural History in Manhattan.

Interpretation of handwriting: "I think case must be that one generation should have as many living as now. To do this and to have as many species in same genus (as is) requires extinction . Thus between A + B the immense gap of relation. C + B the finest gradation. B+D rather greater distinction. Thus genera would be formed. Bearing relation" (next page begins) "to ancient types with several extinct forms". Text in Darwin's format:

36
(text)
I think 
(tree graph) 
B
D C 
1 A
(text on the side)
Case must be that one
generation then should be
as many living as now.
To do this & to have many species in
same genus (as is) requires extinction.
(text below tree graph)
Thus between A & B immense
gap of relation. C & B the 
finest gradation, B & D
rather greater distinction.
Thus genera would be
formed. — bearing relation

(The text continues outside the image:)(page 37) to ancient types. — with several extinct forms for if each species an ancient (1) is capable of making 13 recent forms, twelve of the contemporarys must have left no offspring at all, so as to keep number of species constant. — With respect to extinction we can easy see that variety of ostrich, Petise may not be well adapted, and thus perish out, or on other hand like Orpheus being favourable (page 38) many might be produced. — This requires principle that the permanent varieties produced by inter confined breeding & changing circumstances are continued & produced according to the adaptation of such circumstances, & therefore that death of species is a consequence (contrary to what would appear from America). Source of this text: User fileunderwater at https://biology.stackexchange.com/questions/46481/darwins-first-sketch-of-a-phylogenetic-tree , consulted on 26 April 2021.