SPAKE HALF HOURS

Otago Witness, Issue 1891, 1 May 1890, Page 37

SPAKE HALF HOURS

By F. A. Joseph.

DISTRIBUTION OF ANIMALS AND

PLANTS.

The older theories relating to the distribution of animals and plants over the globe supposed that the different groups had been created in the particular regions in whidh they are found, the later theories suppose that both animals and plants had. a centre of origin^ and that from a few simple fo'raia that first appeared in these centres the varied life of the globe has been derived. This is the only rational explanation that can be given based upon the theory of development, and in support of this contention there can be advanced a considerable amount of argument. Whichever theory We attempt to prove is beset with difficulties— how it came , to pass, for example, that animals and piants were carried across wide intervening expanses of ocean to continents and islands far tero'oved from the supposed centre of origin. We find on comparing ,£li9 geographical divisions known as the Old World and the New World that considerable diversity is displayed by the life they contain — a difference so great as to lend countenance to the special creation theory. Yet there is a difficulty that presents itself at the outset, and that is the fact that the animals and plants of the Old World and the New World present so marked a difference that they would almost appear to represent totally different creation! The,re is nd particular region of the did World; climatically considered, whose ,£oiinterpart cannot be found in the Ne^ World, and yet the life of these corresponding regions is widely diverse. There is, in fact, a peculiarly American type of animal and plant life that cannot be wholly accounted for on climatal and iJhysico,l oohditionp. If we compare certain dissevered regions in ijhe southern hemisphere, as Australia, South Africa, and the western portion of South America, between the parallels 25 and 35, we find regions very closely resembling each other in all their conditions ; yet, in spite of this, no more dissimilar fauna and flora can be found. Now, it is contended by the adherents of the development theory that if the animals and plants found in these regions were specially created for them, why do they not more closely resemble each other 'I They explain the differences by the theory of hereditary descent with variation. The variation, or tendency to Vary, that aniiHals and plants everywhere display, they affirm, is sufficient to account for the marked difference of types that have been completely separated for a long duration of time. As a matter of fact the animals and plants of South America south of latitude 3B and north of 25, although separated by a space of 10 degrees, are mUch more closely related than are those of Australia and Africa in regions Where the climate corresponds. As large areas of watei form the barriers separating land animals and plants, so continental areas constitute the barriers separating the marine fauna and flora; consequently we find that the marine inhabitants of the eastern shore of South America differ widely from those of the western shore, although, according to Gunther, a great authority on the subject, about 30 per cent, of the fishes are the same on opposite sides of the Isthmus of Panama. This fact has led naturalists to believe that the isthmus was once open. Westward df South America there is a wide expanse of ocean with not an island or halting place' for animals, so that when we finally come to the islands of the Pacific we encounter a totally different fauna and flora. Three distinct marine faunas are therefore found in parallel lines not far from each other. Passing still further to the westward over another expanse of ocean> but in this case studded with islands or continuous coasts, till after passing over a hemisphere we come to the shores of Africa ; yet over this wide region there are no distinct faunas, as is the case where wide unbroken expanses of ocean rule. There are numerous facts which might be collected bearing upon the geographical distribution of animals and plants, and most of these facts might be construed to support the theory of discent with variation. Every naturalist is struck by the manner in which successive groups of beings, specifically distinct yet nearly related, replace each other in passing from north to south. For instance, " he hears from closely allied yet distinct kinds of birds notes nearly similar, and sees their nests similarly constructed but not quite alike, with eggs coloured in nearly the same manner." The American ostrich is in reality a bird differing essentially from the true or African ostrich. Now it is found that the rhea (ostrich) inhabiting the plains near the Strait of Magellan differs from the rhea occurring on the plains of La Plata. On these same t>lains the European rabbit and hare are replaced by two animals of similar habits and belonging to the same order of rodents, but they plainly display an American type of structure. If we explore the lofty heights of the Cordillera we find an alpine species of one of these rodents— the bizcacha ; if we examine the waters we do not find the musk rat or the beaver, but these are replaced by distinctly American types of rodents— the coy pu and copybara. If again we explore the islands off the American coast, although they differ essentially in geological structure, we find that all of the animals are of distinctly American type. And if yet again we explore the geological strata for extirjet forms of animal life, we find that these too were of a discinctive American type. In all these facts we see distinct evidence of a close organic bond, exerting its influence throughout space and time, over the same areas of land and water, irrespective of physical conditions — and that bond is inheritance.

The characteristic differences of the inhabitants of separate regions are due to modification through variation and natural selection, probably enough in a subordinate degree to the influence of different physical conditions. Herbert Spencer defines life as the continuous adjustment of internal conditions to external conditions ; the adjustment being continuous, we need not marvel therefore on finding that organisms having a common parental origin vary considerably, as generation succeeds generation, when the

physical conditions are dissimilar. The great diversity of animal life, which to us is the ri^ost tiot^ceabte, especially in animals lieiongihg to the same class or type, is explicable on the theory of hereditary descent with modification acting throughout long periods of time and over wide regions of space. Thus it is that the high importance of barriers comes into play in checking migration, and tinie is all that is needed to account for the differences displayed by animate and plants in isolated regions even although having a commtin Centre of origin. It # _is only on this assumption that w6 rJah account for.;th'e occurrence _ of sections of genera, whole geiiera, bi hibh famines" .in confined areas, as is often the case. Accord* ing to the theory of hereditary descent with modification, the different species of a genus, although inhabiting regions of the world widely separated, must have originally proceeded from the same source, as they evidently have descended from the same progenitor. At first sight it appears difficrilt to accept tills theory; as the barriers often separating species of the sortie! genus seem to be i rnpassable ; yet a fuller considerai?*?*i of , the subject will show that even these barriers $re riot totally insurmountable. We are thus brought face io fadft rntjh the question : Whether the life or the globe na's* had its origin in one or more centres, or whether species were not created where we find them occurring naturally 1 It will be my endeavour, therefore, to attempt to account for the geographical distribution of animals and plants, in accordance with the views of those who believe in the theory of development;