On the principles, and by the light of the facts, now briefly summarised, we have been able, in the present chapter, to indicate how natural selection acts, how divergence of character is set up, how adaptation to conditions at various periods of life has been effected, how it is that low forms of life continue to exist, what kind of circumstances are most favourable to the formation of new species, and, lastly, to what extent the advance of organisation to higher types is produced by natural selection. We will now pass on to consider some of the more important objections and difficulties which have been advanced by eminent naturalists. FOOTNOTES: [Footnote 37: _Origin of Species_, p. 71.] [Footnote 38: Yarrell's _British Birds_, fourth edition, vol. iii. p. 77.] [Footnote 39: _Origin of Species_, p. 89.] [Footnote 40: _Nature_, vol. xxx. p. 30.] CHAPTER VI DIFFICULTIES AND OBJECTIONS Difficulty as to smallness of variations--As to the right variations occurring when required--The beginnings of important organs--The mammary glands--The eyes of flatfish--Origin of the eye--Useless or non-adaptive characters--Recent extension of the region of utility in plants--The same in animals--Uses of tails--Of the horns of deer--Of the scale-ornamentation of reptiles--Instability of non-adaptive characters--Delboeuf's law--No "specific" character proved to be useless--The swamping effects of intercrossing--Isolation as preventing intercrossing--Gulick on the effects of isolation--Cases in which isolation is ineffective. In the present chapter I propose to discuss the more obvious and often repeated objections to Darwin's theory, and to show how far they affect its character as a true and sufficient explanation of the origin of species. The more recondite difficulties, affecting such fundamental questions as the causes and laws of variability, will be left for a future chapter, after we have become better acquainted with the applications of the theory to the more important adaptations and correlations of animal and plant life.