One core notion of Darwin is that there is no such thing in nature as a species (neither race nor gender). There are only individuals. We group them together by means of similarity for our epistemic convenience, but on a purely heuristic basis. This is a profoundly anti-platonic outlook that was not uncommon at his time (Leibniz and Buffon also had argued for something similar, but apparently not really understood the significance), but not the scientific mainstream. Many people (like Kant, for example) argued that the nature of species is essentially unchangeable. And if the nature of a species is fixed, it cannot originate through incremental changes, but only all at once, through an act of creation.

This is one core notion of Darwinism that has received little attention, but its significance is tremendous: now the emergence of species has nothing to do with the change of an abstract 'nature' that unites a certain group of organisms but is rather a great continuum of forms that successively develop. (That too was not unheard of and a widespread idea often called 'the great chain of being'.)

Now you add three other, rather trivial ideas, namely "children are a bit different from their parents" (i.e. mutation), "children are also quite similar to their parents" (i.e. inheritance), and also the idea that not all organisms manage to successfully procreate ("the struggle for life"), and you get the basic tenets of Darwinism. Nothing here was entirely new by itself, but what was new was rather the way he combined it: Darwin effectively fuses theories of the transformation of species and the origins of species together, and he might have been the first person to properly understand the consequences of the idea that there are only individuals in nature.

There had been other theories of adaptation, i.e. the transformation of species beforehand, most famously Lamarck. But Lamarck failed to explain the origin of species, as he still held on to something like a common 'essence' of a species of which only contingent properties would change: giraffes would grow longer necks during their phase of adaptation, because longer necks allowed them to access their food better; but for Lamarck, they could never, say, change the number of legs or eyes or their reproductory organs. For Lamarck, it was incomprehensible that mammals could have developed from reptilians (dinosaurs), but not for Darwin.

So people usually stuck to creationism with some very basic form of adaptationism, often centered around some rather vague notion of 'germs' and 'predispositions' that would unfold differently in different climates and then become unchangeable and inheritable - this, for example, was used for the explanation of the different 'races' of human beings (by authors such as Blumenbach, Buffon, Kant) or for the ontogenetic development of animals (think of Boerhave, Haller, Maupertuis, etc.).

Also worth mentioning is that only in the 19th-century people properly understood the age of the earth - until about 1800 it was still a frequent assumption that the earth would be only 6000 years old (example: Kant), albeit other theories existed (example: Buffon). Only in the 19th-century theories developed by people like Cuvier or Lyell gained traction, who could argue on geological grounds that earth must have been millions of years old. (Speaking very broadly here, exceptions are found easily, even Augustine did argue that the age of creation must have been undefined 'aeons', but as far as I can see most scientists either avoided this question or held on to the idea of a very young earth.) This idea of a very old age of earth made the idea of very, very slight differences between generations leading to rather significant differences millions of years much more plausible, something that would have been difficult to comprehend for older scientists like Blumenbach who would have had difficulties conceiving of such an enormous age of the earth. So in a sense, geology helped pave the way for Darwin.

One might want to add that Darwin's evolutionary theory was still lacking and only somewhat found to be complete after Ernst Mayr integrated it with the Mendelian theory of inheritance - before that, the drivers of mutation and inheritance were simply assumed to be unknown forces of nature (aforementioned germs and predispositions, occasionally also just called drives or forces).

Technically still around was also the Aristotelian idea of spontaneous generation, i.e. that animals would simply self-generate in an appropriate environment - a rotting apple would produce a worm, for example. This is less common in the 17th century onwards, but few people bother to refute it and I have never seen anyone using it to explain the emergence of larger species, like horses or cows.

A good source on the innovation of Darwin would be Wolfgang Lefèvre: "Die Entstehung der biologischen Evolutionstheorie", Frankfurt: Suhrkamp 2009. Ernst Mayr's books on evolution and the growth of biological theory are good as well. For the debate concerning the age of the world see Paolo Rossi: "The Dark Abyss of Time", Chicago: U of Chicago Press 1987. A popular introduction to pre-Darwinian theories of adaptation and creation is Keith Thompson: "Before Darwin: Reconciling God and Nature", New Haven: Yale, 2005 (published under a different name in the UK).

Edited for spelling, typos, clarity, grammar etc.

Edit 2: I have to take care of kids, if there are questions, I'll be back at the computer a few hours later tonight.

One of the people that Darwin could have taken some hints from was his Grand Father. Erasmus Darwin He was one of the first people from the UK to speak on the subject of evolution. In 1794 he published Zoonomia. He also speaks about the idea of a common ancestor. Other things that he would add to the conversation would be sexual selection. "The final course of this contest among males seems to be, that the strongest and most active animal should propagate the species which should thus be improved". However his view points did not take hold and he was even mocked.