The geological relationships that exist between the current fauna and the extinct fauna of South America, as well as certain facts concerning the distribution of the organized beings that inhabit this continent, deeply impressed me during my voyage aboard the ship Beagle, in my capacity as a naturalist.

These facts, as will be seen in the subsequent chapters of this volume, seem to shed some light on the origin of species – the mystery of mysteries – to use the expression of one of the greatest philosophers. Upon my return to England in 1837, I believed that by patiently accumulating all the facts related to this subject and examining them from all perspectives, I might eventually be able to elucidate this question.

After five years of persistent work, I drafted some notes; later, in 1844, I summarized these notes in the form of a memoir, where I indicated the results that seemed to offer some degree of probability; since that time, I have constantly worked towards the same goal. I hope the reader will excuse me for going into such personal details, and if I do so, it is to prove that no decision of mine has been made lightly (...).