Darwin's On the Origin of Species. Daniel Duzdevich
of living things. A typical study may involve a bacterial strain coaxed into producing large quantities of a protein native to yeast, a markedly different type of organism. Bacteria and yeast are very distantly related, having diverged from a common ancestor billions of years ago. But that common ancestor already possessed the most fundamental, and the most important, molecular qualities of life – including DNA as the hereditary material, and the machinery needed to manufacture proteins based on information carried by a gene. These characteristics are so important for the very existence of a cell that they cannot be altered in any essential way, so they are passed on from generation to generation, all the way through to the cells growing in a couple of flasks in a laboratory. This continuity explains why bacteria can correctly read a gene from yeast in order to make protein: the two species converse in one molecular language, as all living things do. The phenomenon of shared characteristics applies to successive groupings of the evolutionary tree. All animal embryos, for example, share a common molecular and cellular system for development, inherited from a common ancestor. Darwin observed that such commonalities cluster together in this way, and he recognized their origin: it is common ancestry, which makes the modern science of biology possible, and understandable, and sometimes wonderfully shocking.
ACKNOWLEDGMENTS
MANY PEOPLE CONTRIBUTED TO THE DEVELOPMENT OF THIS project, and I am indebted to all of them. A series of conversations with Walter Bock were formative. Two anonymous reviewers critiqued an early draft and helped me improve the text. Ashley Hennen made insightful and important comments on a later version. The impeccable copyediting work of Jill R. Hughes polished the final draft. Myles Marshall designed the inspired cover and reworked the tree diagram. Thomas Cole created and maintains the companion website. I am especially grateful to Olivia Judson for writing the foreword and for thoughtful discussions. Eric Greene and members of his laboratory were very supportive as I worked to complete the manuscript. Robert Sloan, Nancy Lightfoot, and many others at Indiana University Press made this book a reality.
DARWIN’S
ON THE ORIGIN OF SPECIES
INTRODUCTION
WHEN ON BOARD HMS BEAGLE AS NATURALIST, I WAS STRUCK by the distribution of South America’s organisms and the geological relationships between its past and present inhabitants. These observations seemed to me to illuminate that mystery of mysteries: the origin of species. After my return home, it occurred to me, in 1837, that this question may be clarified by patiently accumulating and reflecting on all sorts of relevant facts. Following five years’ work I began to speculate on the subject and drew up some short notes, which I enlarged in 1844 into a sketch of probable conclusions. Since then I have steadily pursued the same object. I hope the reader will excuse me for entering on these personal details; I just want to show that I have not been hasty in coming to a decision.
My work is nearly finished, but I will require two or three more years to complete it, and as my health is far from strong I have been urged to publish this abstract. I was further prompted to do this because last year Alfred Russel Wallace, who is now studying the natural history of the Malay Archipelago, sent me a memoir in which he arrives at conclusions very similar to mine. He requested that I forward it to Sir Charles Lyell. Lyell and Dr. Hooker, who both knew of my work – Dr. Hooker having read my 1844 sketch – honored me by advising publication of extracts from my own manuscript alongside Mr. Wallace’s excellent piece. Both appear in the third volume of the Journal of the Linnean Society.
This abstract is necessarily imperfect. I cannot here provide references and must trust the reader’s confidence in my accuracy. Errors have no doubt crept in, although I have tried to use only reliable authorities. I give only general conclusions with a few illustrative examples, which I hope will suffice. And I entirely appreciate the necessity of describing all the information upon which I ground my conclusions in a future work. I am well aware that little is discussed to which additional observations could not be given, often leading to apparently contradictory conclusions. A fair assessment can only be reached by a full statement of the facts and balancing arguments on both sides of each issue; this cannot possibly be done here.
I regret that a lack of space prevents acknowledgment of the generous assistance I have received from many naturalists, some of them personally unknown to me. However, I will not pass up this opportunity to thank Dr. Hooker, who has aided me over the past fifteen years in every possible way with his excellent judgment and large stores of knowledge.
A naturalist considering the origin of species and reflecting on the affinities among organisms, their embryological relationships, their geographic distribution, geological succession, and other factors might conclude that each species had not been created independently but had descended as varieties do from other species. But even if such a conclusion were well founded, it would be unsatisfactory until it were shown how the innumerable species inhabiting the earth have been modified to a perfection of structure and coadaptation that justly excites our admiration. Naturalists refer to external conditions, such as climate and food supply, as the only possible cause of variation. This may be true in one very limited sense, as we will see, but it’s absurd to claim, for example, that the woodpecker’s feet, tail, beak, and tongue have become so well adapted to catch insects under tree bark simply due to external conditions. Or consider the mistletoe, which draws nourishment from certain trees, has seeds that must be transported by specific birds, and has flowers with separate sexes requiring particular insects to bring pollen from one flower to the other. It is equally absurd to claim that the structure of this parasitic plant, with its relationships to distinct organisms, results from habits or volition or external conditions. The author of the Vestiges of the Natural History of Creation would presumably argue that after an indeterminate number of generations some bird had given birth to a perfectly formed woodpecker and some plant had brought forth a perfectly formed mistletoe. But this assumption is no explanation, leaving the coadaptations of organisms to one another and their physical environments untouched.
So it is very important to learn how modification and coadaptation happen. As I began my observations, it seemed that a careful study of domesticated plants and animals would provide the best chance for insight into this problem. I have not been disappointed; our knowledge of variation under domestication, though imperfect, invariably affords the best and safest clue to these and other perplexing problems. I suggest that this field is highly valuable, though it has commonly been neglected by naturalists.
I therefore devote the first chapter of this abstract to variation under domestication. I demonstrate that a large amount of hereditary modification is at least possible and, perhaps more importantly, that humans have caused huge changes in domesticated plants and animals through the selection and accumulation of slight successive variations. I then briefly discuss the variability of species in the wild. This topic could only have been treated properly by long catalogs of facts, but I nevertheless discuss the circumstances favorable to variation. The third chapter treats the struggle for existence among all organisms, which follows inevitably from their ability to proliferate geometrically: the doctrine of Malthus applied to all living things. Because many more individuals of each species are born than can possibly survive, any individual possessing even a slightly favorable variation enjoys a better chance of surviving the complex and sometimes fluctuating environment and is naturally selected. The principle of inheritance ensures that a selected variety will tend to propagate its new and modified form.
This fundamental subject of natural selection is treated at length in the fourth chapter, where I discuss how it often causes extinction of less improved life forms and induces divergence of character. In the following chapter I address the complex and poorly understood rules of variation and correlated growth. In the four succeeding chapters I present the most obvious and serious challenges to the theory: (1) how a simple organism or simple organ can be changed and perfected into something highly developed or elaborately constructed, (2) the mental power of animals (instinct), (3) the infertility of species but fertility of varieties when crossed (hybridism), and (4) the imperfection of the geological record. Then I consider the geological succession of organisms through time; in the eleventh and twelfth chapters, their geographic distribution; in the thirteenth, their classification, based on affinities in both embryonic and fully developed