There were three objections that Darwin's fellow naturalists could make, and Darwin was very sensitive to each and every one of them, and his diaries specifically mention these three:

• Religious: The book of Genesis is explicit that the organisms we see around us had all been created, just as they are now, by God. Included here is the perception of humans as 'special'.
  
  
• Cultural: Descartes says its only the initial conditions - evolution requires ever changing conditions (like in Geology) and so evolution really is inconsistent with the mechanical universe framework.
  
  
• Scientific: No naturalist claimed to have ever seen / observed a new species evolve out of another. And even if it were true, what could be the explanation for the change? What is the mechanism that drives evolution? ...that explains for example the complexity of the human eye?

Wilberforce Criticism (1860:

• First, then, he ... declares that he applies his scheme of the action of the principle of natural selection to man himself, as well as to the animals around him.
  • Now, we must say at once, and openly, that such a notion is absolutely incompatible ... with single expressions in the word of God..
  • Man's derived supremacy over the earth; man's power of articulate speech; man's gift of reason; man's free will and responsibility; man's fall and man's redemption; the incarnation of the Eternal Son; the indwelling of the Eternal Spirit---all are equally and utterly irreconcilable with the degrading notion of the brute origin of him who was created in the image of God, and redeemed by the Eternal Son ...
  • Equally inconsistent, too, is Mr. Darwin's daring notion of man's further development into some unknown extent of powers and shape, and size, through natural selection acting through that long vista of ages which He casts mistily over the earth upon the most favored individuals of His species...

David Hull, The Spectator, 1860:

But I cannot conclude without expressing my detestation of the theory, because of its unflinching materialism; -- because it has deserted the inductive track, the only track that leads to physical truth; --because it utterly repudiates final causes [shades of Aristotle], and therby indicates a demoralized understanding on the part of its advocates.

St. George Mivart:

The Incompetency of 'Natural Selection' to account for the Incipient Stages of Useful Structures." If this phrase sounds like a mouthful, consider the easy translation: we can readily understand how complex and full developed structures work and owe their maintenance and preservation to natural selection---a wing, an eye, the resemblance of a bittern [heron] to a branch or of an insect to a stick or dead leaf. But how do you get from nothing to such an elaborate something if evolution must proceed through a long sequence of intermediate stages, each favored by natural selection? You can't fly with 2% of a wing or gain much protection from an iota's similarity with a potentially concealing piece of vegetation. How, in other words, can natural selection explain these incipient stages of structures that can only be used (as we now observe them) in much more elaborated form?

The Darwin response:

• In any growing population food resources will eventually become scarce and a struggle for existence ensues. There will be competition for scarce resource and MOST competitors will fail.
  
  
• It is only those which are best adapted which will survive and this is a natural (and what he calls Natural Selection).
  
  
• Within populations there are variations and some varaitions are more advantageous than others. Individuals with those variations allow for a favored group to emerge.
  
  
• Over time this favored group, by virute of tts accumulated advantages, becomes so distinct from the norm that the two groups can no longer breed together, the 'favored race' will have become in fact a new species. (but this takes thousands of generations).