[{54}] See Origin, Ed. i. pp. 62, 63, vi. p. 77, where similar reference is made to De Candolle; for Malthus see Origin, p. 5.
[{55}] This may possibly refer to the amount of destruction going on. See Origin, Ed. i. p. 68, vi. p. 84, where there is an estimate of a later date as to death-rate of birds in winter. “Calculate robins” probably refers to a calculation of the rate of increase of birds under favourable conditions.
[{56}] In the Origin, Ed. i. pp. 64, 65, vi. p. 80, he instances cattle and horses and certain plants in S. America and American species of plants in India, and further on, as unexpected effects of changed conditions, the enclosure of a heath, and the relation between the fertilisation of clover and the presence of cats (Origin, Ed. i. p. 74, vi. p. 91).
[{57}] Origin, Ed. i. p. 74, vi. p. 91. “It has been observed that the trees now growing on ... ancient Indian mounds ... display the same beautiful diversity and proportion of kinds as in the surrounding virgin forests.”
[{58}] The simile of the wedge occurs in the Origin, Ed. i. p. 67; it is deleted in Darwin’s copy of the first edition: it does not occur in Ed. vi.
[{59}] In a rough summary at the close of the Essay, occur the words:—“Every creature lives by a struggle, smallest grain in balance must tell.”
[{60}] Cf. Origin, Ed. i. p. 77, vi. p. 94.
[{61}] This is a repetition of what is given at p. 6.
[{62}] Compare Origin, Ed. i. p. 41, vi. p. 47. “I have seen it gravely remarked, that it was most fortunate that the strawberry began to vary just when gardeners began to attend closely to this plant. No doubt the strawberry had always varied since it was cultivated, but the slight varieties had been neglected.”
[{63}] Here we have the two types of sexual selection discussed in the Origin, Ed. i. pp. 88 et seq., vi. pp. 108 et seq.