This text, the subject of so much political and social controversy in the U.S., is actually just a nice read. First, and in surprising contradiction to all the God versus science panic, he presumes an "act of creation," which to me, implies that his theory is based on the idea of a Creator.
Here are a few quotes from Chapter Two in which he discusses acts of creation:
No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation.
The term species thus comes to be a mere useless abstraction, implying and assuming a separate act of creation.
On the other hand, if we look at each species as a special act of creation, there is no apparent reason why more varieties should occur in a group having many species, than in one having few.
He then spends considerable time discussing and thinking about the anomalies in domesticated animals. The domesticated duck, for example, has larger leg bones and smaller wing bones, which he attributes to more time spent walking and less spent flying. He notes that many domesticated animals develop, over numerous generations floppy ears, which he speculates is to due to loss of musculature from attention to potential dangers -- a skill domesticated, human-protected animals no longer require.
There are anomalies among domesticated animals because in his day, there was a theory that left without interference from breeders, animals would "revert" to their "pure" ... presumably original created forms. Of course, Darwin observes that this isn't true and that one can observe that a continuation of development of various different attributes is normal.
This book is a delightful read because as Darwin wonders why the animals and plants grow, procreate and develop as they do, it is easy to follow the natural curiosity of his mind.
He obviously spent considerable time with people who specialized in breeding animals and those who modified plant types to improve their strength, color, taste, size, etc.
Following all this discussion on domestication, he then ventures into a discussion about the need for an external source for reproduction and concludes that while some species of plants and worms can breed themselves, they can not do so indefinitely and will require another specimen to breed with to ensure the strength of the offspring. Of course, today, we understand clearly the genetic drawbacks of inbreeding, but Darwin was exploring the concepts and coming to, obviously, very solid conclusions.
He talks about the success of animals and plants that are non-Native to a given area and the potential to disrupt the lives of native plants and animals in the area immigrated to.
I don't think this is necessarily a book just for scientists and academicians, this is a great read if you just want to pass a rainy weekend in contemplation of the world we live in.