Continuity of Succession of Forms in Life and Time. Geographical Distribution of Ancient Faunae and Florae.
In any given locality, the succession of living forms may appear to be interrupted by numerous breaks -- the associated species in each fossiliferous bed being quite distinct from those above and those below them. But the tendency of all palaeontological investigation is to show that these breaks are only apparent, and arise from the incompleteness of the series of remains which happens to have been preserved in any given locality. As the area over which accurate geological investigations have been carried on extends, and as the fossiliferous rocks found in one locality fill up the gaps left in another, so do the abrupt demarcations between the faunae and florae of successive epochs disappear -- a certain proportion of the genera and species of every period, great or small, being found to be continued for a longer or shorter time into the next succeeding period. It is evident, in fact, that the changes in the living population of the globe which have taken place during its history, have been effected, not by the sudden replacement of one set of living beings by another, but by a process of slow and gradual introduction of new species, accompanied by the circumstances of the older forms.
It is a remarkable circumstance, that in all parts of the globe in which fossiliferous rocks have yet been examined, the successive terms of the series of living forms which have thus succeeded one another are analogous. The life of the Mesozoic epoch is everywhere characterized by the abundance of some groups of species of which no trace is to be found in either earlier or later formations; and the like is true of the Palaeozoic epoch. Hence it follows, not only that there has been a succession of species, but that the general nature of that succession has been the same all over the globe; and it is on this ground that fossils are so important to the geologist as marks, of the relative age of rocks.
The determination of the morphological relations of the species which have thus succeeded one another is a problem of profound importance and difficulty, the solution of which, however, is already clearly indicated. For, in several cases, it is possible, to show that, in the same geographical area, a form A, which existed during a certain geological epoch, has been replaced by another form B, at a later, by a third form C. when these forms, A, B, and C, are compared together they are found to be organized upon the same plan, and to be very similar even in most of the details of their structure; but B differs from A by a slight modification of some of its parts, which modification is carried to a still greater extent in C.
In other words, A, B, and C differ from one another in the same fashion as the earlier and later stages of the embryo of the same animals differ; and in successive epochs we the group presenting that progressive specialization which characteristics the development of the individual. Clear evidence that this progressive specialization of structure has actually occurred has a yet been obtained in only a few cases (e.g. Equidae, Crocodilia), and these are confined to the highest and most complicated, forms of life; while it is demonstrable that, even as reckoned by geological time, the process must have been exceedingly slow.
Among the lower and less complicated forms the evidence of progressive modifications, furnished by comparison of the orders with the latest forms, is slight, or absent; and some of these have certainly persisted, with very little change, from extremely ancient times to the present day. It is as important to recognize the fact that certain forms of life have thus persisted, as it is to admit that others have undergone progressive modification.
It has been said that the successive terms in the series of living forms are analogous in all parts of the globe. But the species which constitute the corresponding or homotaxic terms in the series, in different localities, are not identical. And, though the imperfection of our knowledge at present precludes positive assertion, there is every reason to believe that geographical provinces have existed throughout the period during which organic remains furnish us with evidence of the existence of life. The wide distribution of certain Palaezoic forms does not militate against this view; for the recent investigations into the nature of the deep-sea fauna have shown that numerous Crustacea, Echinodermata, and other invertebrate animals, have as wide a distribution now as their analogues possessed in the Silurian epoch.
Read the rest of this article:
Biology - Table of Contents