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THE ANCIENT
LIFE-HISTORY
OF THEĀ EARTH

Chapter 6:

THE BIOLOGICAL RELATIONS OF FOSSILS.


Not only have fossils, as we have seen, a most important bearing upon the sciences of Geology and Physical Geography, but they have relations of the most complicated and weighty character with the numerous problems connected with the study of living beings, or in other words, with the science of Biology. To such an extent is this the case, that no adequate comprehension of Zoology and Botany, in their modern form, is so much as possible without some acquaintance with the types of animals and plants which have passed away. There are also numerous speculative questions in the domain of vital science, which, if soluble at all, can only hope to find their key in researches carried out on extinct organisms. To discuss fully the biological relations of fossils would, therefore, afford matter for a separate treatise; and all that can be done here is to indicate very cursorily the principal points to which the attention of the palæontological student ought to be directed.

In the first place, the great majority of fossil animals and plants are "extinct"—that is to say, they belong to species which are no longer in existence at the present day. So far, however, from there being any truth in the old view that there were periodic destructions of all the living beings in existence upon the earth, followed by a corresponding number of new creations of animals and plants, the actual facts of the case show that the extinction of old forms and the introduction of new forms have been processes constantly going on throughout the whole of geological time. Every species seems to come into being at a certain definite point of time, and to finally disappear at another definite point; though there are few instances indeed, if there are any, in which our present knowledge would permit us safely to fix with precision the times of entrance and exit. There are, moreover, marked differences in the actual time during which different species remained in existence, and therefore corresponding differences in their "vertical range," or, in other words, in the actual amount and thickness of strata through which they present themselves as fossils. Some species are found to range through two or even three formations, and a few have an even more extended life. More commonly the species which begin in the commencement of a great formation die out at or before its close, whilst those which are introduced for the first time near the middle or end of the formation may either become extinct, or may pass on into the next succeeding formation. As a general rule, it is the animals which have the lowest and simplest organisation that have the longest range in time, and the additional possession of microscopic or minute dimensions seems also to favour longevity. Thus some of the Foraminifera appear to have survived, with little or no perceptible alteration, from the Silurian period to the present day; whereas large and highly-organised animals, though long-lived as individuals, rarely seem to live long specifically, and have, therefore, usually a restricted vertical range. Exceptions to this, however, are occasionally to be found in some "persistent types," which extend through a succession of geological periods with very little modification. Thus the existing Lampshells of the genus Lingula are little changed from the Lingulœ which swarmed in the Lower Silurian seas; and the existing Pearly Nautilus is the last descendant of a clan nearly as ancient. On the other hand, some forms are singularly restricted in their limits, and seem to have enjoyed a comparatively brief lease of life. An example of this is to be found in many of the Ammonites—close allies of the Nautilus—which are often confined strictly to certain zones of strata, in some cases of very insignificant thickness.

Of the causes of extinction amongst fossil animals and plants, we know little or nothing. All we can say is, that the attributes which constitute a species do not seem to be intrinsically endowed with permanence, any more than the attributes which constitute an individual, though the former may endure whilst many successive generations of the latter have disappeared. Each species appears to have its own life-period, its commencement, its culmination, and its gradual decay; and the life-periods of different species may be of very different duration.

From what has been said above, it may be gathered that our existing species of animals and plants are, for the most part, quite of modern origin, using the term "modern" in its geological acceptation. Measured by human standards, the majority of existing animals (which are capable of being preserved as fossils) are known to have a high antiquity; and some of them can boast of a pedigree which even the geologist may regard with respect. Not a few of our shellfish are known to have commenced their existence at some point of the Tertiary period; one Lampshell (Terebratulina caput-serpentis) is believed to have survived since the Chalk; and some of the Foraminifera date, at any rate, from the Carboniferous period. We learn from this the additional fact that our existing animals and plants do not constitute an assemblage of organic forms which were introduced into the world collectively and simultaneously, but that they commenced their existence at very different periods, some being extremely old, whilst others may be regarded as comparatively recent animals. And this introduction of the existing fauna and flora was a slow and gradual process, as shown admirably by the study of the fossil shells of the Tertiary period. Thus, in the earlier Tertiary period, we find about 95 per cent of the known fossil shells to be species that are no longer in existence, the remaining 5 per cent being forms which are known to live in our present seas. In the middle of the Tertiary period we find many more recent and still existing species of shells, and the extinct types are much fewer in number; and this gradual introduction of forms now living goes on steadily, till, at the close of the Tertiary period, the proportions with which we started may be reversed, as many as 90 or 95 per cent of the fossil shells being forms still alive, while not more than 5 per cent may have disappeared.

All known animals at the present day may be divided into some five or six primary divisions, which are known technically as "sub-kingdoms." Each of these sub-kingdoms [9] may be regarded as representing a certain type or plan of structure, and all the animals comprised in each are merely modified forms of this common type. Not only are all known living animals thus reducible to some five or six fundamental plans of structure, but amongst the vast series of fossil forms no one has yet been found—however unlike any existing animal—to possess peculiarities which would entitle it to be placed in a new sub-kingdom. All fossil animals, therefore, are capable of being referred to one or other of the primary divisions of the animal kingdom. Many fossil groups have no closely-related group now in existence; but in no case do we meet with any grand structural type which has not survived to the present day.

[Footnote 9: In the Appendix a brief definition is given of the sub-kingdoms, and the chief divisions of each are enumerated.]

The old types of life differ in many respects from those now upon the earth; and the further back we pass in time, the more marked does this divergence become. Thus, if we were to compare the animals which lived in the Silurian seas with those inhabiting our present oceans, we should in most instances find differences so great as almost to place us in another world. This divergence is the most marked in the Palæozoic forms of life, less so in those of the Mesozoic period, and less still in the Tertiary period. Each successive formation has therefore presented us with animals becoming gradually more and more like those now in existence; and though there is an immense and striking difference between the Silurian animals and those of to-day, this difference is greatly reduced if we compare the Silurian fauna with the Devonian; that again with the Carboniferous; and so on till we reach the present.

It follows from the above that the animals of any given formation are more like those of the next formation below, and of the next formation above, than they are to any others; and this fact of itself is an almost inexplicable one, unless we believe that the animals of any given formation are, in part at any rate, the lineal descendants of the animals of the preceding formation, and the progenitors, also in part at least, of the animals of the succeeding formation. In fact, the palæontologist is so commonly confronted with the phenomenon of closely-allied forms of animal life succeeding one another in point of time, that he is compelled to believe that such forms have been developed from some common ancestral type by some process of "evolution." On the other hand, there are many phenomena, such as the apparently sudden introduction of new forms throughout all past time, and the common occurrence of wholly isolated types, which cannot be explained in this way. Whilst it seems certain, therefore, that many of the phenomena of the succession of animal life in past periods can only be explained by some law of evolution, it seems at the same time certain that there has always been some other deeper and higher law at work, on the nature of which it would be futile to speculate at present.

Not only do we find that the animals of each successive formation become gradually more and more like those now existing upon the globe, as we pass from the older rocks into the newer, but we also find that there has been a gradual progression and development in the types of animal life which characterise the geological ages. If we take the earliest-known and oldest examples of any given group of animals, it can sometimes be shown that these primitive forms, though in themselves highly organised, possessed certain characters such as are now only seen in the young of their existing representatives. In technical language, the early forms of life in some instances possess "embryonic" characters, though this does not prevent them often attaining a size much more gigantic than their nearest living relatives. Moreover, the ancient forms of life are often what is called "comprehensive types"—that is to say, they possess characters in combination such as we nowadays only find separately developed in different, groups of animals. Now, this permanent retention of embryonic characters and this "comprehensiveness" of structural type are signs of what a zoologist considers to be a comparatively low grade of organisation; and the prevalence of these features in the earlier forms of animals is a very striking phenomenon, though they are none the less perfectly organised so far as their own type is concerned. As we pass upwards in the geological scale, we find that these features gradually disappear, higher and ever higher forms are introduced, and "specialisation" of type takes the place of the former comprehensiveness. We shall have occasion to notice many of the facts on which these views are based at a later period, and in connection with actual examples. In the meanwhile, it is sufficient to state, as a widely-accepted generalisation of palæontology, that there has been in the past a general progression of organic types, and that the appearance of the lower forms of life has in the main preceded that of the higher forms in point of time.



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