Chapter V: Distribution in Time and Space

The origin of Vertebrates, and how far back in time they extend, is unknown. The earliest fishes were in all probability devoid of hard parts and traces of their existence can scarcely be expected to be found. The hypothesis that they may be derived from the early Crustaceans, or Arachnids, is chiefly based on the somewhat striking resemblance which the mailed fishes of the Silurian period (Ostracodermi) bear to the Arthropods of that remote time, a resemblance, however, very superficial and regarded by most morphologists as an interesting example of mimetic resemblance--whatever this term may be taken to mean. The minute denticles known as conodonts, which first appear in the Ordovician, were once looked upon as teeth of Cyclostomes, but their histological structure does not afford any support to the identification and they are now generally dismissed altogether from the Vertebrates. As a compensation the Lower Silurian of Russia has yielded small teeth or spines which seem to have really belonged to fishes, although their exact affinities are not known (_Palaeodus_ and _Archodus_ of J. V. Rohon).

It is not until we reach the Upper Silurian that satisfactory remains of unquestionable fishes are found, and here they suddenly appear in a considerable variety of forms, very unlike modern fishes in every respect, but so highly developed as to convince us that we have to search in much earlier formations for their ancestors. These Upper Silurian fishes are the _Coelolepidae_, the _Ateleaspidae_, the _Birkeniidae_, the _Pteraspidae_, the _Tremataspidae_ and the _Cephalaspidae_, all referred to the Ostracophori. The three last types persist in the Devonian, in the middle of which period the Osteolepid Crossopterygii, the Dipneusti and the Arthrodira suddenly appear. The most primitive Selachian (_Cladoselache_), the Acanthodian Selachians (_Diplacanthidae_), the Chimaerids (_Ptyctodus_), and the Palaeoniscid ganoids (_Chirolepis_) appear in the Upper Devonian, along with the problematic _Palaeospondylus_.

In the Carboniferous period, the Ostracophori and Arthrodira have disappeared, the Crossopterygii and Dipneusti are still abundant, and the Selachians (_Pleuracanthus_, Acanthodians, truesharks) and Chondrostean ganoids (_Palaeoniscidae_ and _Platysomidae_) are predominant. In the Upper Permian the Holostean ganoids (_Acanthophorus_) make their appearance, and the group becomes dominant in the Jurassic and the Lower Cretaceous. In the Trias, the Crossopterygii and Dipneusti dwindle in variety and the _Ceratodontidae_ appear; the Chondrostean and Holostean ganoids are about equally represented, and are supplemented in the Jurassic by the first, annectant representatives of the Teleostei (_Pholidophoridae_, _Leptolepidae_). In the latter period, the Holostean ganoids are predominant, and with them we find numerous Cestraciont sharks, some primitive skates (_Squatinidae_ and _Rhinobatidae_), Chimaerids and numerous Coelacanthid crossopterygians.

The fish-fauna of the Lower Cretaceous is similar to that of the Jurassic, whilst that of the Chalk and other Upper Cretaceous formations is quite modern in aspect, with only a slight admixture of Coelacanthid crossopterygians and Holostean ganoids, the Teleosteans being abundantly represented by _Elopidae_, _Albulidae_, _Halosauridae_, _Scopelidae_ and _Berycidae_, many being close allies of the present inhabitants of the deep sea. At this period the spiny-rayed Teleosteans, dominant in the seas of the present day, made their first appearance.

With the Eocene, the fish-fauna has assumed the essential character which it now bears. A few Pycnodonts survive as the last representatives of typically Mesozoic ganoids, whilst in the marine deposits of Monte Bolca (Upper Eocene) the principal families of living marine fishes are represented by genera identical with or more or less closely allied to those still existing; it is highly remarkable that forms so highly specialized as the sucking-fish or remoras, the flat-fish (_Pleuronectidae_), the Pediculati, the Plectognaths, &c., were in existence, whilst in the freshwater deposits of North America _Osteoglossidae_ and _Cichlidae_ were already represented. Very little is known of the freshwater fishes of the early Tertiaries. What has been preserved of them from the Oligocene and Miocene shows that they differed very slightly from their modern representatives. We may conclude that from early Tertiary times fishes were practically as they are at present. The great hiatus in our knowledge lies in the period between the Cretaceous and the Eocene.

At the present day the Teleosteans are in immense preponderance, Selachians are still well represented, the Chondrostean ganoids are confined to the rivers and lakes of the temperate zone of the northern hemisphere (_Acipenseridae_, _Polyodontidae_), the Holostean ganoids are reduced to a few species (_Lepidosteus_, _Amia_) dwelling in the fresh waters of North America, Mexico and Cuba, the Crossopterygians are represented by the isolated group _Polypteridae_, widely different from any of the known fossil forms, with about ten species inhabiting the rivers and lakes of Africa, whilst the Dipneusti linger in Australia (_Neoceratodus_), in South America (_Lepidosiren_), and in tropical Africa (_Protopterus_). The imperfections of the geological record preclude any attempt to deal with the distribution in space as regards extinct forms, but several types, at present very restricted in their habitat, once had a very wide distribution. The _Ceratodontidae_, for instance, of which only one species is now living, confined to the rivers of Queensland, has left remains in Triassic, Rhaetic, Jurassic and Cretaceous rocks of Europe, North America, Patagonia, North and South Africa, India and Australia; the _Amiidae_ and _Lepidosteidae_ were abundant in Europe in Eocene and Miocene times; the _Osteoglossidae_, now living in Africa, S.E. Asia and South America, occurred in North America and Europe in the Eocene.

In treating of the geographical distribution of modern fishes, it is necessary to distinguish between fresh-water and marine forms. It is, however, not easy to draw a line between these categories, as a large number of forms are able to accommodate themselves to either fresh or salt water, whilst some periodically migrate from the one into the other. On the whole, fishes may be roughly divided into the following categories:--

I. Marine fishes. A. shore-fishes; B. pelagic fishes; C. deep-sea
fishes.

II. Brackish-water fishes.

III. Fresh-water fishes.

IV. Migratory fishes. A. anadromous (ascending fresh waters to spawn);
B. catadromous (descending to the sea to spawn).

About two-thirds of the known recent fishes are marine. Such are nearly all the Selachians, and, among the Teleosteans, all the _Heteromi_, _Pediculati_ and the great majority of _Apodes_, _Thoracostei_, _Percesoces_, _Anacanthini_, _Acanthopterygii_ and _Plectognathi_. All the _Crossopterygii_, _Dipneusti_, _Opisthomi_, _Symbranchii_, and nearly all the _Ganoidei_ and _Ostariophysi_ are confined to fresh-water.

The three categories of marine fishes have thus been defined by Günther:--

"1. _Shore Fishes_--that is, fishes which chiefly inhabit parts of the
sea in the immediate neighbourhood of land either actually raised
above, or at least but little submerged below, the surface of the
water. They do not descend to any great depth,--very few to 300
fathoms, and the majority live close to the surface. The distribution
of these fishes is determined, not only by the temperature of the
surface water, but also by the nature of the adjacent land and its
animal and vegetable products,--some being confined to flat coasts
with soft or sandy bottoms, others to rocky and fissured coasts,
others to living coral formations. If it were not for the frequent
mechanical and involuntary removals to which these fishes are exposed,
their distribution within certain limits, as it no doubt originally
existed, would resemble still more that of freshwater fishes than we
find it actually does at the present period.

2. _Pelagic Fishes_--that is, fishes which inhabit the surface and
uppermost strata of the open ocean, and approach the shores only
accidentally or occasionally (in search of prey), or periodically (for
the purpose of spawning). The majority spawn in the open sea, their
ova and young being always found at a great distance from the shore.
With regard to their distribution, they are still subject to the
influences of light and the temperature of the surface water; but they
are independent of the variable local conditions which tie the shore
fish to its original home, and therefore roam freely over a space
which would take a freshwater or shore fish thousands of years to
cover in its gradual dispersal. Such as are devoid of rapidity of
motion are dispersed over similarly large areas by the oceanic
currents, more slowly than the strong swimmers, but not less surely.
An accurate definition, therefore, of their distribution within
certain areas equivalent to the terrestrial regions is much less
feasible than in the case of shore fishes.

3. _Deep-Sea Fishes_--that is, fishes which inhabit such depths of the
ocean that they are but little or not at all influenced by light or
the surface temperature, and which, by their organization, are
prevented from reaching the surface stratum in a healthy condition.
Living almost under identical tellurian conditions, the same type, the
same species, may inhabit an abyssal depth under the equator as well
as one near the arctic or antarctic circle; and all that we know of
these fishes points to the conclusion that no separate horizontal
regions can be distinguished in the abyssal fauna, and that no
division into bathymetrical strata can be attempted on the base of
generic much less of family characters."

A division of the world into regions according to the distribution of the shore-fishes is a much more difficult task than that of tracing continental areas. It is possible perhaps to distinguish four great divisions: the Arctic region, the Atlantic region, the Indo-Pacific region and the Antarctic region. The second and third may be again subdivided into three zones: Northern, Tropical and Southern. This appears to be a more satisfactory arrangement than that which has been proposed into three zones primarily, each again subdivided according to the different oceans. Perhaps a better division is that adopted by D. S. Jordan, who arranges the littoral fishes according to coast lines; we then have an East Atlantic area, a West Atlantic, an East Pacific and a West Pacific, the latter including the coasts of the Indian Ocean. The tropical zone, whatever be the ocean, is that in which fishes flourish in greatest abundance and where, especially about coral-reefs, they show the greatest variety of bizarre forms and the most gorgeous coloration. The fish-fauna of the Indo-Pacific is much richer than that of the Atlantic, both as regards genera and species.

As regards the Arctic and Antarctic regions, the continuity or circumpolar distribution of the shore fishes is well established. The former is chiefly characterized by its Cottids, Cyclopterids, Zoarcids and Gadids, the latter by its Nototheniids. The theory of bipolarity receives no support from the study of the fishes.

Pelagic fishes, among which we find the largest Selachians and Teleosteans, are far less limited in their distribution, which, for many species, is nearly world-wide. Some are dependent upon currents, but the great majority being rapid swimmers able to continue their course for weeks, apparently without the necessity of rest (many sharks, scombrids, sword-fishes), pass from one ocean into the other. Most numerous between the tropics, many of these fishes occasionally wander far north and south of their habitual range, and there are few genera that are at all limited in their distribution.

Deep-sea fishes, of which between seven hundred and eight hundred species are known, belong to the most diverse groups and quite a number of families are exclusively bathybial (_Chlamydoselachidae_, _Stomiatidae_, _Alepocephalidae_, _Nemichthyidae_, _Synaphobranchidae_, _Saccopharyngidae_, _Cetomimidae_, _Halosauridae_, _Lipogenyidae_, _Notacanthidae_, _Chiasmodontidae_, _Icosteidae_, _Muraenolepididae_, _Macruridae_, _Anomalopidae_, _Podatelidae_, _Trachypteridae_, _Lophotidae_, _Ceratiidae_, _Gigantactinidae_). But they are all comparatively slight modifications of the forms living on the surface of the sea or in the shallow parts, from which they may be regarded as derived. In no instance do these types show a structure which may be termed archaic when compared with their surface allies. That these fishes are localized in their vertical distribution, between the 100-fathoms line, often taken as the arbitrary limit of the bathybial fauna, and the depth of 2750 fathoms, the lowest point whence fishes have been procured, there is little doubt. But our knowledge is still too fragmentary to allow of any general conclusions, and the same applies to the horizontal distribution. Yet the same species may occur at most distant points; as these fishes dwell beyond the influence of the sun's rays, they are not affected by temperature, and living in the Arctic zone or under the equator makes little difference to them. A great deal of evidence has been accumulated to show the gradual transition of the surface into the bathybial forms; a large number of surface fishes have been met with in deep water (from 100 to 500 fathoms), and these animals afford no support to Alexander Agassiz's supposition of the existence of an azoic zone between the 200-fathoms line and the bottom.

Brackish-water fishes occur also in salt and fresh water, in some localities at least, and belong to various groups of Teleosteans. Sticklebacks, gobies, grey mullets, blennies are among the best-known examples. The facility with which they accommodate themselves to changes in the medium in which they live has enabled them to spread readily over very large areas. The three-spined stickleback, for instance, occurs over nearly the whole of the cold and temperate parts of the northern hemisphere, whilst a grey mullet (_Mugil capito_) ranges without any appreciable difference in form from Scandinavia and the United States along all the Atlantic coasts to the Cape of Good Hope and Brazil. It would be hardly possible to base zoo-geographical divisions on the distribution of such forms.

The fresh-water fishes, however, invite to such attempts. How greatly their distribution differs from that of terrestrial animals has long ago been emphasized. The key to their mode of dispersal is, with few exceptions, to be found in the hydrography of the continents, latitude and climate, excepting of course very great altitudes, being inconsiderable factors, the fish-fauna of a country deriving its character from the headwaters of the river-system which flows through it. The lower Nile, for instance, is inhabited by fishes bearing a close resemblance to, or even specifically identical with, those of tropical Africa, thus strikingly contrasting with the land-fauna of its banks. The knowledge of the river-systems is, however, not sufficient for tracing areas of distribution, for we must bear in mind the movements which have taken place on the surface of the earth, owing to which present conditions may not have existed within comparatively recent times, geologically speaking; and this is where the systematic study of the aquatic animals affords scope for conclusions having a direct bearing on the physical geography of the near past. It is not possible here to enter into the discussion of the many problems which the distribution of fresh-water fishes involves; we limit ourselves to an indication of the principal regions into which the world may be divided from this point of view. The main divisions proposed by Günther in the 9th edition of the _Encyclopædia Britannica_ still appear the most satisfactory. They are as follows:--