Читать книгу Darwin's On the Origin of Species - Daniel Duzdevich - Страница 9

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 states; and in the last chapter I give a brief summary of the work and a few concluding remarks.

Given our ignorance of the relationships among organisms, it is not surprising that much remains to be explained about the origin of species. Who can explain why one species is numerous with a wide range while a related species is rare with a narrow range? And yet such questions are important because their answers explain the present state and future modifications and success of all organisms. We know even less about the relationships among the innumerable past inhabitants of the earth. Although much remains obscure, and will long remain obscure, the most deliberate study and dispassionate judgment of which I am capable have dissuaded me from the view commonly held by naturalists, and previously held by me, that each species has been independently created. I am fully convinced that species are mutable and that species within a genus are linearly descended from some usually extinct species, in the same way that a variety of a given species is a descendant of that species. I am also convinced that natural selection has been the main, though not exclusive, means of modification.