Horses: The Story of the Horse Family in the Modern World and Through Sixty Million Years of History.

by George Gaylord Simpson

For scope, clarity and detail this book is a classic case history of the evolutionary process. Dr. Simpson, a distringuished scientist and noteworthy author, first looks at the horse today, discussing the many modern breeds and wild varieties - including the wild ass and the zebra. He then explores in detail the lineage of the horse and its descent from Hyracotherium or what is sometimes known as Eohippus. He shows how horse evolution has been deciphered from the fossil record and how this record contributes to a fuller understanding of the development of life on earth.

Genres: Evolution

247 pages, Hardcover

First published August 5, 1971

About the author

George Gaylord Simpson, Ph.D. (Geology, Yale University, 1926), was Professor of Geosciences at the University of Arizona from 1968 until his retirement in 1982. Previously was Curator of the Museum of Comparative Zoology at Harvard University 1959–1970, Curator of the Department of Geology and Paleontology at the American Museum of Natural History 1945–1959, and Professor of Zoology at Columbia University.

He was awarded the Linnean Society of London's prestigious Darwin-Wallace Medal in 1958. Simpson also received the Royal Society's Darwin Medal 'In recognition of his distinguished contributions to general evolutionary theory, based on a profound study of palaeontology, particularly of vertebrates,' in 1962. In 1966, Simpson received the Golden Plate Award of the American Academy of Achievement.

Reviews

[Steven H · Rating 5 out of 5]

THE FAMED PALEONTOLOGIST LOOKS AT HORSES, AND THEIR EVOLUTION

George Gaylord Simpson (1902-1984) was one of the most influential American paleontologists of the twentieth century. His major works include 'Tempo and Mode in Evolution,' 'The Major Features of Evolution,' 'This View of Life: The World of an Evolutionist,' 'The Meaning of Evolution,' etc. [NOTE: page numbers below refer to the 324-page Doubleday paperback edition.]

He wrote in the Introduction to this 1951 book, “This is a book about horses as animals… the center of interest is to be the animal and not its use… An important reason why horses are so fascinating … is that they are such representative animals. From horses we may learn not only about the horse itself but also about animals in general, indeed about ourselves and about life as a whole, its history and characteristics. The aim of this book is not only to present the members of the horse family, but also to present an introduction to the study of animals and of life, with horses providing concrete and excellent examples...” (Pg. xxxii) He adds, “The story of the horse family provides one of the best means for studying the how and why of evolution.” (Pg. xxxiv)

He points out, “This early work [by T.H. Huxley and Vladimir Kovalevsky] carried the story almost as far as was possible in Europe. The fact is, as we now know, that the central lines of horse evolution did not occur on that continent. Huxley and Kovalevsky were perfectly right in listing these genera as successive stages of evolution among the relatives of the horses, but the three earlier forms were not directly ancestral to Equus and belong on three different offshoots from the direct ancestry. The first was a side branch that evolved in the Old World and the other two had migrated successively from the distant region where direct descent to Equus was occurring. This sort of thing happened frequently in the course of evolution and is responsible for series of structural stages which lack the actual ancestral-descendant transitional links. The transition can only be filled in by preservation and discovery of fossils in the region where the main evolutionary line was developing.” (Pg. 117)

He observes, “After Miohippus the history of the horse became considerably more complicated. It is no longer a matter of following what appears to be a single great lineage as in the American HyracotheriumOrohippusEpihippusMesohippusMiohippus sequence. Now the horses tend once more to split up into various different groups which evolved separately and in somewhat different ways… It is sad that this introduces possible confusion into the story, but there is not much point in criticizing nature for something that happened millions of years ago. It would also be foolish to try to ignore the complications, which did occur and which are a very important part of the record. Most accounts of horse evolution have simplified the situation to the point of deliberately falsifying it. Let us here face the facts of the record and try to follow them as clearly as possible but without misleading omissions.” (Pg. 166-168)

He notes, “The extinction of horses over the whole of North and South America, where they had roamed in vast herds during the Pleistocene, is one of the most mysterious episodes of animal history. There is no doubt about the fact, but the reason for it is doubtful to say the least. There has been no lack of speculation and a dozen possible explanations have been suggested, but all of these lack evidence and none is really satisfactory… This seems at present one of the situations in which we must be humble and honest and admit that we do not know the answer… The general cause of extinction … must have been the occurrence of changes to which the animal populations could not adapt themselves. But what precisely were those changes? And why could not the horses … adapt to them, while many other groups of animals could?” (Pg. 198, 200)

He states, “The most convincing feature of the horse family as evidence of evolution is that throughout most of their history the variations demonstrated in collections of successive ages overlap: the ancestral and descendant populations intergrade completely in this way. There is still one gap where overlap is not quite complete… and there are some other points where more material is needed on the level of relatively unimportant details in regard to species, but the sequence is now so well represented that it leaves no doubt whatsoever about the main fact in the mind of anyone who studies it extensively. The history of the horse family is still one of the clearest and most convincing for showing that organisms really have evolved, for demonstrating that, so to speak, an onion can turn into a lily” (Pg. 224)

He says, “Increase in the relative size of the teeth, change in type of pattern, complication of pattern, and increase in crown height all occurred in horses and were the main changes in their teeth. They did not all occur in the same way or at the same time in different lines if descent, or at a constant rate at any one line… It should also be emphasized that, although these adaptive features explain the main changes in the teeth, they do not explain all the fluctuations and changes in details. Minor differences in pattern characteristic of closely allied genera and species seem in some cases to have no particular adaptive value and to have arisen at random. These irregularities in horse evolution have commonly been overlooked or undervalued, and the bearing of the evidence on the principles of evolution has therefore sometimes been misunderstood.” (Pg. 243-244)

He summarizes, “The evolution of the horse family was definitely not orthogenetic… There was, for instance, no constant and over-all increase in size. Most recent horses are larger than most ancient horses, but when the history is examined in detail it shows that there have been long periods when no increase in size occurred and also several branches of horse evolution in which the animals became markedly smaller, not larger. The feet did not steadily change from four toes to three and then to one… toe reduction… from three to one was not a continuation of a previous trend… but was a change in evolutionary direction, a new development that occurred at just one time and in only one group of horses.

"And so it goes for all the changes that have occurred in the history of the family; not one of them shows the constant, guided change in a single direction that is demanded by the theory of orthogenesis [i.e., evolution proceeds in a unilinear fashion]. In reality, the horse family goes far to disprove that theory, and when other supposed examples of orthogenesis are examined with similar care they, too, are found to be opposed to the reality of orthogenesis in any strict sense.” (Pg. 270-271)

Although more than 70 years old, this book still contains much illuminating information about the evolutionary directions of horses.