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On the Law which has Regulated by Alfred Russel Wallace 1855 Geographical Distribution Dependent on Geologic Changes. EVERY naturalist who has directed his attention to the subject of the geographical distribution of animals and plants, must have been interested in the singular facts which it presents. Many of these facts are quite different from what would have been anticipated, and have hitherto been considered as highly curious, but quite inexplicable. None of the explanations attempted from the time of Linnaeus are now considered at all satisfactory; none of them have given a cause sufficient to account for the facts known at the time, or comprehensive enough to include all the new facts which have since been, and are daily being added. Of late years, however, a great light has been thrown upon the subject by geological investigations, which have shown that the present state of the earth and of the organisms now inhabiting it, is but the last stage of a long and uninterrupted series of changes which it has undergone, and consequently, that to endeavour to explain and account for its present condition without any reference to those changes (as has frequently been done) must lead to very imperfect and erroneous conclusions. The facts proved by geology are briefly these:- That during
an immense, but unknown period, the surface of the earth has
undergone successive changes; land has sunk beneath the ocean,
while fresh land has risen up from it; mountain chains have been
elevated; islands have been formed into continents, and continents
submerged till they have become islands; and these changes have
taken place, not once merely, but perhaps hundreds, perhaps thousands
of times:- That all these operations have been more or less continuous,
but unequal in their progress, and during the whole series the
organic life of the earth has Now, taking this as a fair statement of the results of geological inquiry, we see that the present geographical distribution of life upon the earth must be the result of all the previous changes, both of the surface of the earth itself and of its inhabitants. Many causes, no doubt, have operated of which we must ever remain in ignorance, and we may, therefore, expect to find many details very difficult of explanation, and in attempting to give one, must allow ourselves to call into our service geological changes which it is highly probable may have occurred, though we have no direct evidence of their individual operation. The great increase of our knowledge within the last twenty years, both of the present and past history of the organic world, has accumulated a body of facts which should afford a sufficient foundation for a comprehensive law embracing and explaining them all, and giving a direction to new researches. It is about ten years since the idea of such a law suggested itself to the writer of this essay, and he has since taken every opportunity of testing it by all the newly-ascertained facts with which he has become acquainted, or has been able to observe himself. These have all served to convince him of the correctness of his hypothesis. Fully to enter into such a subject would occupy much space, and it is only in consequence of some views having been lately promulgated, he believes, in a wrong direction, that he now ventures to present his ideas to the public, with only such obvious illustrations of the arguments and results as occur to him in a place far removed from all means of reference and exact information. A Law deduced from well-known Geographical and Geological Facts. The following propositions in Organic Geography and Geology give the main facts on which the hypothesis is founded. GEOGRAPHY GEOLOGY This law agrees with, explains and illustrates all the facts connected with the following branches of the subject:- 1st. The system of natural affinities. 2nd. The distribution of animals and plants in space. 3rd. The same in time, including all the phaenomena of representative groups, and those which Professor Forbes supposed to manifest polarity. 4th. The phaenomena of rudimentary organs. We will briefly endeavour to show its bearing upon each of these. The Form of a true system of Classification determined by this Law. If the law above enunciated be true, it follows that the natural series of affinities will also represent the order in which the several species came into existence, each one having had for its immediate antitype a closely allied species existing at the time of its origin. It is evidently possible that two or three distinct species may have had a common antitype, and that each of these may again have become the antitypes from which other closely allied species were created. The effect of this would be, that so long as each species has had but one new species formed on its model, the line of affinities will be simple, and may be represented by placing the several species in direct succession in a straight line. But if two or more species have been independently formed on the plan of a common antitype, then the series of affinities will be compound, and can only be represented by a forked or many branched line. Now, all attempts at a Natural classification and arrangement of organic beings show, that both these plans have obtained in creation. Sometimes the series of affinities can be well represented for a space by a direct progression from species to species or from group to group, but it is generally found impossible so to continue. There constantly occur two or more modifications of an organ or modifications of two distinct organs, leading us on to two distinct series of species, which at length differ so much from each other as to form distinct genera or families. These are the parallel series or representative groups of naturalists, and they often occur in different countries, or are found fossil in different formations. They are said to have an analogy to each other when they are so far removed from their common antitype as to differ in many important points of structure, while they still preserve a family resemblance. We thus see how difficult it is to determine in every case whether a given relation is an analogy or an affinity, for it is evident that as we go back along the parallel or divergent series, towards the common antitype, the analogy which existed between the two groups becomes an affinity. We are also made aware of the difficulty of arriving at a true classification, even in a small and perfect group;- in the actual state of nature it is almost impossible, the species being so numerous and the modifications of form and structure so varied, arising probably from the immense number of species which have served as antitype for the existing species, and thus produced a complicated branching of the lines of affinity, as intricate as the twigs of a gnarled oak or the vascular system of the human body. Again, if we consider that we have only fragments of this vast system, the stem and main branches being represented by extinct species of which we have no knowledge, while a vast mass of limbs and boughs and minute twigs and scattered leaves is what we have to place in order, and determine the true position each originally occupied with regard to the others, the whole difficulty of the true Natural System of classification becomes apparent to us. We shall thus find ourselves obliged to reject all those systems of classification which arrange species or groups in circles, as well as those which fix a definite number for the divisions of each group. The latter class have been very generally rejected by naturalists, as contrary to nature, notwithstanding the ability with which they have been advocated; but the circular system of affinities seems to have obtained a deeper hold, many eminent naturalists having to some extent adopted it. We have, however, never been able to find a case in which the circle has been closed by a direct and close affinity. In most cases a palpable analogy has been substituted, in others the affinity is very obscure or altogether doubtful. The complicated branching of the lines of affinities in extensive groups must also afford great facilities for giving a show of probability to any such purely artificial arrangements. Their death-blow was given by the admirable paper of the lamented Mr. Strickland, published in the "Annals of Natural History," in which he so cleverly showed the true synthetical method of discovering the Natural System. Geographical Distribution of Organisms. If we now consider the geographical distribution of animals and plants upon the earth, we shall find all the facts beautifully in accordance with, and readily explained by, the present hypothesis. A country having species, genera, and whole families peculiar to it, will be the necessary result of its having been isolated for a long period, sufficient for many series of species to have been created on the type of pre-existing ones, which, as well as many of the earlier-formed species, have become extinct, and thus made the groups appear isolated. If in any case the antitype had an extensive range, two or more groups of species might have been formed, each varying from it in a different manner, and thus producing several representative or analogous groups. The Sylviadae of Europe and the Sylvicolidae of North America, the Heliconidae of South America and the Euploeas of the East, the group of Trogons inhabiting Asia, and that peculiar to South America, are examples that may be accounted for in this manner. Such phaenomena as are exhibited by the Galapagos Islands,
which contain little groups of plants and animals peculiar to When a range of mountains has attained a great elevation, and has so remained during a long geological period, the species of the two sides at and near their bases will be often very different, representative species of some genera occurring, and even whole genera being peculiar to one side, as is remarkably seen in the case of the Andes and Rocky Mountains. A similar phaenomena occurs when an island has been separated from a continent at a very early period. The shallow sea between the Peninsula of Malacca, Java, Sumatra and Borneo was probably a continent or large island at an early epoch, and may have become submerged as the volcanic ranges of Java and Sumatra were elevated. The organic results we see in the very considerable number of species of animals common to some or all of these countries, while at the same time a number of closely allied representative species exist peculiar to each, showing that a considerable period has elapsed since their separation. The facts of geographical distribution and of geology may thus mutually explain each other in doubtful cases, should the principles here advocated be clearly established. In all those cases in which an island has been separated from a continent, or raised by volcanic or coralline action from the sea, or in which a mountain-chain has been elevated in a recent geological epoch, the phaenomena of peculiar groups or even of single representative species will not exist. Our own island is an example of this, its separation from the continent being geologically very recent, and we have consequently scarcely a species which is peculiar to it; while the Alpine range, one of the most recent mountain elevations, separates faunas and floras which scarcely differ more than may be due to climate and latitude alone. The series of facts alluded to in Proposition (3), of closely
allied species in rich groups being found geographically near
each other, is most striking and important. Mr. Lovell Reeve
has well exemplified it in his able and interesting paper on
the Geological Distribution of the Forms of Life. The phaenomena of geological distribution are exactly analogous
to those of geography. Closely allied species are found High Organization of very ancient Animals consistent with this Law. Much discussion has of late years taken place on the question,
whether the succession of life upon the globe has been from a Returning to the analogy of a branching tree, as the best
mode of representing the natural arrangement of species and their Objections to Forbes' Theory of Polarity. The hypothesis of polarity, recently put forward by Professor
Edward Forbes to account for the abundance of generic forms at
a very early period and at present, while in the intermediate
epochs there is a gradual diminution and impoverishment, till
the minimum occurred at the confines of the Palaeozoic and Secondary
epochs, appears to us quite unnecessary, as the facts may be
readily accounted for on the principles already laid down. Between
the Palaeozoic and Neozoic periods of Professor Forbes, there
is scarcely a species in common, and the greater part of the
genera and families also disappear to be replaced by new ones.
It is almost universally admitted that such a change in the organic
world must have occupied a vast period of time. Of this interval
we have no record; probably because the whole area of the early
formations now exposed to our researches was elevated at the
end of the Palaeozoic period, and remained so through the interval
required for the organic changes which resulted in the fauna
and flora of the Secondary period. The records of this interval
are buried beneath the ocean which covers three-fourths of the
globe. Now it appears highly probable that a long period of quiescence
or stability in the physical conditions of a district would be
most favourable to the existence of organic life in the greatest
abundance, both as regards individuals and also as to variety
of species and generic group, just as we now find that the places
best adapted to the rapid growth and increase of individuals
also contain the greatest profusion of species and the greatest
variety of forms,- the tropics in comparison with the temperate
and arctic regions. On the other hand, it seems no less probable
that a change in the physical conditions of a district, even
small in amount if rapid, or even gradual if to a great amount,
would be highly unfavourable to the existence of individuals,
might cause the extinction of many species, and would probably
be equally unfavourable to the creation of new ones. In this
too we may find an analogy with the present state of our earth,
for it has been shown to be the violent extremes and rapid changes
of physical conditions, rather than the actual mean state in
the temperate and frigid zones, which renders them less prolific
than the tropical regions, as exemplified by the great distance
beyond the tropics to which tropical forms penetrate when the
climate is equable, and also by the richness in species and forms
of tropical mountain regions which principally differ from the
temperate zone in the uniformity of their climate. However this
may be, it seems a fair assumption that during a period of geological
repose the new species which we know to have been created would
have appeared, that the creations would then exceed in number
the extinctions, and therefore the number of species would increase.
In a period of geological activity, on the other hand, it seems
probable that the extinctions might exceed the creations, and
the number of species consequently diminish. That such effects
did take place in connexion with the causes to which we have
imputed them, is shown in the case of the Coal formation, the
faults and contortions of which show a period of great activity
and violent convulsions, and it is in the formation immediately
succeeding this that the poverty of forms of life is most apparent.
We have then only to suppose a long period of somewhat similar
action during the vast unknown interval at the termination of
the Palaeozoic period, and then a decreasing violence or rapidity
through the Secondary period, to allow for the gradual repopulation
of the earth with varied forms, and the whole of the facts are
explained. We thus have a clue to the increase of the forms of
life during certain periods, and their decrease during I would also venture to suggest some reasons against the very nature of the theory of Professor Forbes. Our knowledge of the organic world during any geological epoch is necessarily very imperfect. Looking at the vast numbers of species and groups that have been discovered by geologists, this may be doubted; but we should compare their numbers not merely with those that now exist upon the earth, but with a far larger amount. We have no reason for believing that the number of species on the earth at any former period was much less than at present; at all events the aquatic portion, with which geologists have most acquaintance, was probably often as great or greater. Now we know that there have been many complete changes of species; new sets of organisms have many times been introduced in place of old ones which have become extinct, so that the total amount which have existed on the earth from the earliest geological period must have borne about the same proportion to those now living, as the whole human race who have lived and died upon the earth, to the population at the present time. Again, at each epoch, the whole earth was no doubt, as now, more or less the theatre of life, and as the successive generations of each species died, their exuviae and preservable parts would be deposited over every portion of the then existing seas and oceans, which we have reason for supposing to have been more, rather than less, extensive than at present. In order then to understand our possible knowledge of the early world and its inhabitants, we must compare, not the area of the whole field of our geological researches with the earth's surface, but the area of the examined portion of each formation separately with the whole earth. For example, during the Silurian period all the earth was Silurian, and animals were living and dying, and depositing their remains more or less over the whole area of the globe, and they were probably (the species at least) nearly as varied in different latitudes and longitudes as at present. What proportion do the Silurian districts bear to the whole surface of the globe, land and sea (for far more extensive Silurian districts probably exist beneath the ocean than above it), and what portion of the known Silurian districts has been actually examined for fossils? Would the area of rock actually laid open to the eye be the thousandth or the ten-thousandth part of the earth's surface? Ask the same question with regard to the Oolite or the Chalk, or even to particular beds of these when they differ considerably in their fossils, and you may then get some notion of how small a portion of the whole we know. But yet more important is the probability, nay almost the
certainty, that whole formations containing the records of vast Rudimentary Organs Another important series of facts, quite in accordance with,
and even necessary deductions from, the law now developed, are Conclusion It has now been shown, though most briefly and imperfectly, how the law that "Every species has come into existence coincident both in time and space with a pre-existing closely allied species," connects together and renders intelligible a vast number of independent and hitherto unexplained facts. The natural system of arrangement of organic beings, their geographical distribution, their geological sequence, the phaenomena of representative and substituted groups in all their modifications, and the most singular peculiarities of anatomical structure, are all explained and illustrated by it, in perfect accordance with the vast mass of facts which the researches of modern naturalists have brought together, and, it is believed, not materially opposed to any of them. It also claims a superiority over previous hypotheses, on the ground that it not merely explains, but necessitates what exists. Granted the law, and many of the most important facts in Nature could not have been otherwise, but are almost as necessary deductions from it, as are the elliptic orbits of the planets from the law of gravitation. Sarawak, Borneo, Feb 1855
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