others. And it is these empirical laws which are embodied
and expressed in a natural classification.
II. Distribution.
Living beings occupy certain portions of the surface of the earth, inhabiting either the dry land or the fresh or salt waters, or being competent to maintain their existence in either. In any given locality, it is found that those different media are inhabited by different kinds of living beings ; and that the same medium, at different heights in the air and at different depths in the water, has different living inhabitants.
Moreover, the living populations of localities which differ considerably in latitude, and hence in climate, always present considerable differences. But the converse proposition is not true ; that is to say, localities which differ in longitude, even if they resemble one another in climate, often have very dissimilar faunæ and floræ.
It has been discovered by careful comparison of local faunae and floras that certain areas of the earth s surface are inhabited by groups of animals and plants which are not found elsewhere, and which thus characterize each of these areas. Such areas are termed Provinces of Distribution. There is no parity between these provinces in extent, nor in the physical configuration of their boundaries ; and, in reference to existing conditions, nothing can appear to be more arbitrary and capricious than the distribution of living beings.
The study of distribution is not confined to the present order of nature ; but, by the help of geology, the naturalist is enabled to obtain clear, though too fragmentary, evidence of the characters of the faunas and florae of antecedent epochs. The remains of organisms which are contained in the stratified rocks prove that, in any given part of the earth s surface, the living population of earlier epochs was different from that which now exists in the locality ; and that, on the whole, the difference becomes greater the farther we go back in time. The organic remains which are found in the later Cainozoic deposits of any district are always closely allied to those now found in the province of distribution in which that locality is included ; while in the older Cainozoic, the Mesozoic, and the Paleozoic strata, the fossils may be similar to creatures at present living in some other province, or may be altogether unlike any which now exist.
In any given locality, the succession of living forms may appear to be interrupted by numerous breaks the asso ciated species in each fossiliferous bed being quite distinct from those above and those below them. But the ten dency of all palasontological investigation is to show that these breaks are only apparent, and arise from the incom pleteness 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 floras 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 suc ceeding period. It is evident, in fact, that the changes in the living population of the globe which have takenplace 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 extinction of the older forms.
It is a remarkable circumstance, that in all parts of the globs in which fossiliferous rocks have yet been examined, tho 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 ago 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 period ; and that this form B has been replaced, still later, by a third form C. When these forms, A, B, and C, are com pared together they are found to be organized upon the same plan, and to be very similar even in most of the de tails 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 have the group presenting that progressive specialization which characterises the development of the individual. Clear evidence that this progressive specialization of structure has actually occurred has as yet been obtained in only a few cases (e.g., Equidce, Crocodilia), and these are confined to the highest and most complicated forms of life ; while it is demonstrable that, even as reckoned by geolo gical time, the process must have been exceedingly slow. Among the lower and less complicated forms the evi dence of progressive modifications, furnished by compari son of the oldest 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 through out the period during which organic remains furnish us with evidence of the existence of life. The wide distribu tion of certain Palasozoic 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.
III. Physiology.
Thus far living beings have been regarded merely as definite forms of matter, and Biology has presented no considerations of a different order from those which meet the student of Mineralogy. But living things are not only natural bodies, having a definite form and mode of structure, growth, and development. They are machines in action ; and, under this aspect, the phenomena which they present have no parallel in the mineral world.
