[{508}] In the Origin, Ed. i. p. 469, vi. p. 644, Darwin makes a strong statement to this effect.
[{509}] “A grain in the balance will determine which individual shall live and which shall die,” Origin, Ed. i. p. 467, vi. p. 642. A similar statement occurs in the 1842 Essay, [p. 8, note 3].
[{510}] Thus according to the author what is now known as orthogenesis is due to selection.
[{511}] Part II begins with Ch. IV. See the [Introduction], where the absence of division into two parts (in the Origin) is discussed.
[{512}] In the recapitulation in the last chapter of the Origin, Ed. i. p. 475, vi. p. 651, the author does not insist on this point as the weightiest difficulty, though he does so in Ed. i. p. 299. It is possible that he had come to think less of the difficulty in question: this was certainly the case when he wrote the 6th edition, see p. 438.
[{513}] «The following words:» The fauna changes singly «were inserted by the author, apparently to replace a doubtful erasure».
[{514}] This question forms the subject of what is practically a section of the final chapter of the Origin (Ed. i. p. 480, vi. p. 657).
[{515}] Origin, Ed. i. p. 481, vi. p. 659.
[{516}] The discussion on the three species of Rhinoceros which also occurs in the Essay of 1842, p. [48], was omitted in Ch. XIV of the Origin, Ed. i.
[{517}] This corresponds to a paragraph in the Origin, Ed. i. p. 483, vi. p. 662, where it is assumed that animals have descended “from at most only four or five progenitors, and plants from an equal or lesser number.” In the Origin, however, the author goes on, Ed. i. p. 484, vi. p. 663: “Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype.”