[Footnote 5: _The Origin of Species_, p. 53.] [Footnote 6: _The Earth as Modified by Human Action_, p. 51.] [Footnote 7: _The Origin of Species_, p. 56.] [Footnote 8: See _Nature_, vol. xxxi. p. 63.] [Footnote 9: _A Visit to South America_, 1878; also _Nature_, vol. xxxi. pp. 263-339.] [Footnote 10: Still more remarkable is the increase of rabbits both in New Zealand and Australia. No less than seven millions of rabbit-skins have been exported from the former country in a single year, their value being £67,000. In both countries, sheep-runs have been greatly deteriorated in value by the abundance of rabbits, which destroy the herbage; and in some cases they have had to be abandoned altogether.] [Footnote 11: Later observers have proved that two eggs are laid and usually two young produced, but it may be that in most cases only one of these comes to maturity.] [Footnote 12: _Origin of Species_, p. 59. Professor A. Newton, however, informs me that these species do not interfere with one another in the way here stated.] [Footnote 13: Winwood Reade's _Martyrdom of Man,_ p. 520.] [Footnote 14: _Nineteenth Century,_ February 1888, pp. 162, 163.] [Footnote 15: The Kestrel, which usually feeds on mice, birds, and frogs, sometimes stays its hunger with earthworms, as do some of the American buzzards. The Honey-buzzard sometimes eats not only earthworms and slugs, but even corn; and the Buteo borealis of North America, whose usual food is small mammals and birds, sometimes eats crayfish.] CHAPTER III THE VARIABILITY OF SPECIES IN A STATE OF NATURE Importance of variability--Popular ideas regarding it--Variability of the lower animals--The variability of insects--Variation among lizards--Variation among birds--Diagrams of bird-variation--Number of varying individuals--Variation in the mammalia--Variation in internal organs--Variations in the skull--Variations in the habits of Animals--The Variability of plants--Species which vary little--Concluding remarks.