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.
One of the earliest and most often repeated objections was, that it was difficult "to imagine a reason why variations tending in an infinitesimal degree in any special direction should be preserved," or to believe that the complex adaptation of living organisms could have been produced "by infinitesimal beginnings." Now this term "infinitesimal," used by a well-known early critic of the _Origin of Species_, was never made use of by Darwin himself, who spoke only of variations being "slight," and of the "small amount" of the variations that might be selected.

Even in using these terms he undoubtedly afforded grounds for the objection above made, that such small and slight variations could be of no real use, and would not determine the survival of the individuals possessing them.

We have seen, however, in our third chapter, that even Darwin's terms were hardly justified; and that the variability of many important species is of considerable amount, and may very often be properly described as large.

As this is found to be the case both in animals and plants, and in all their chief groups and subdivisions, and also to apply to all the separate parts and organs that have been compared, we must take it as proved that the average _amount_ of variability presents no difficulty whatever in the way of the action of natural selection.