Darwin and Modern Science [133]
True, there are very many unsolved problems, and the discouraged worker is often tempted to believe that the fossils raise more questions than they answer. Yet, on the other hand, the whole trend of the evidence is so strongly in favour of the evolutionary doctrine, that no other interpretation seems at all rational.
To present any adequate account of the palaeontological record from the evolutionary standpoint, would require a large volume and a singularly unequal, broken and disjointed history it would be. Here the record is scanty, interrupted, even unintelligible, while there it is crowded with embarrassing wealth of material, but too often these full chapters are separated by such stretches of unrecorded time, that it is difficult to connect them. It will be more profitable to present a few illustrative examples than to attempt an outline of the whole history.
At the outset, the reader should be cautioned not to expect too much, for the task of determining phylogenies fairly bristles with difficulties and encounters many unanswered questions. Even when the evidence seems to be as copious and as complete as could be wished, different observers will put different interpretations upon it, as in the notorious case of the Steinheim shells. (In the Miocene beds of Steinheim, Wurtemberg, occur countless fresh-water shells, which show numerous lines of modification, but these have been very differently interpreted by different writers.) The ludicrous discrepances which often appear between the phylogenetic "trees" of various writers have cast an undue discredit upon the science and have led many zoologists to ignore palaeontology altogether as unworthy of serious attention. One principal cause of these discrepant and often contradictory results is our ignorance concerning the exact modes of developmental change. What one writer postulates as almost axiomatic, another will reject as impossible and absurd. Few will be found to agree as to how far a given resemblance is offset by a given unlikeness, and so long as the question is one of weighing evidence and balancing probabilities, complete harmony is not to be looked for. These formidable difficulties confront us even in attempting to work out from abundant material a brief chapter in the phylogenetic history of some small and clearly limited group, and they become disproportionately greater, when we extend our view over vast periods of time and undertake to determine the mutual relationships of classes and types. If the evidence were complete and available, we should hardly be able to unravel its infinite complexity, or to find a clue through the mazes of the labyrinth. "Our ideas of the course of descent must of necessity be diagrammatic." (D.H. Scott, "Studies in Fossil Botany", page 524. London, 1900.)
Some of the most complete and convincing examples of descent with modification are to be found among the mammals, and nowhere more abundantly than in North America, where the series of continental formations, running through the whole Tertiary period, is remarkably full. Most of these formations contain a marvellous wealth of mammalian remains and in an unusual state of preservation. The oldest Eocene (Paleocene) has yielded a mammalian fauna which is still of prevailingly Mesozoic character, and contains but few forms which can be regarded as ancestral to those of later times. The succeeding fauna of the lower Eocene proper (Wasatch stage) is radically different and, while a few forms continue over from the Paleocene, the majority are evidently recent immigrants from some region not yet identified. From the Wasatch onward, the development of many phyla may be traced in almost unbroken continuity, though from time to time the record is somewhat obscured by migrations from the Old World and South America. As a rule, however, it is easy to distinguish between the immigrant and the indigenous elements of the fauna.
From their gregarious habits and individual abundance, the history of many hoofed animals is preserved with especial clearness. So
To present any adequate account of the palaeontological record from the evolutionary standpoint, would require a large volume and a singularly unequal, broken and disjointed history it would be. Here the record is scanty, interrupted, even unintelligible, while there it is crowded with embarrassing wealth of material, but too often these full chapters are separated by such stretches of unrecorded time, that it is difficult to connect them. It will be more profitable to present a few illustrative examples than to attempt an outline of the whole history.
At the outset, the reader should be cautioned not to expect too much, for the task of determining phylogenies fairly bristles with difficulties and encounters many unanswered questions. Even when the evidence seems to be as copious and as complete as could be wished, different observers will put different interpretations upon it, as in the notorious case of the Steinheim shells. (In the Miocene beds of Steinheim, Wurtemberg, occur countless fresh-water shells, which show numerous lines of modification, but these have been very differently interpreted by different writers.) The ludicrous discrepances which often appear between the phylogenetic "trees" of various writers have cast an undue discredit upon the science and have led many zoologists to ignore palaeontology altogether as unworthy of serious attention. One principal cause of these discrepant and often contradictory results is our ignorance concerning the exact modes of developmental change. What one writer postulates as almost axiomatic, another will reject as impossible and absurd. Few will be found to agree as to how far a given resemblance is offset by a given unlikeness, and so long as the question is one of weighing evidence and balancing probabilities, complete harmony is not to be looked for. These formidable difficulties confront us even in attempting to work out from abundant material a brief chapter in the phylogenetic history of some small and clearly limited group, and they become disproportionately greater, when we extend our view over vast periods of time and undertake to determine the mutual relationships of classes and types. If the evidence were complete and available, we should hardly be able to unravel its infinite complexity, or to find a clue through the mazes of the labyrinth. "Our ideas of the course of descent must of necessity be diagrammatic." (D.H. Scott, "Studies in Fossil Botany", page 524. London, 1900.)
Some of the most complete and convincing examples of descent with modification are to be found among the mammals, and nowhere more abundantly than in North America, where the series of continental formations, running through the whole Tertiary period, is remarkably full. Most of these formations contain a marvellous wealth of mammalian remains and in an unusual state of preservation. The oldest Eocene (Paleocene) has yielded a mammalian fauna which is still of prevailingly Mesozoic character, and contains but few forms which can be regarded as ancestral to those of later times. The succeeding fauna of the lower Eocene proper (Wasatch stage) is radically different and, while a few forms continue over from the Paleocene, the majority are evidently recent immigrants from some region not yet identified. From the Wasatch onward, the development of many phyla may be traced in almost unbroken continuity, though from time to time the record is somewhat obscured by migrations from the Old World and South America. As a rule, however, it is easy to distinguish between the immigrant and the indigenous elements of the fauna.
From their gregarious habits and individual abundance, the history of many hoofed animals is preserved with especial clearness. So