For some months this surging procession moves northward, sporting in the summertime 

 splendour of the sea. Then, at the end of the year, when this great movement has ceased and 

 the Atlantic waters gradually withdraw, to regain from the tropical sun the heat lost on the 

 way, all these brilliant animals turn on their tracks and move southward. But they never 

 leave the waters which are their essential living space. Conversely, other fishes flee from these 

 warm, salty waters and evade them by moving northwards or by seeking a refuge nearer the 

 coasts. There is thus a biological distinction in the oceans between the fishes of the trans- 

 gressions and the fishes of polar and continental waters. 



To give some idea of the relations between transgressive waters and polar, continental and 

 abyssal waters, imagine that the latter form an immense basin in the middle of which floats 

 a gigantic spot of oil with ceaselessly changing outlines, a spot representing the waters of equa- 

 torial origin. These two disiinct oceanic water masses do not mix with one another, and each 

 carries its own peculiar marine fauna. Because of their stenothermal and stenohaline habits, 

 species that live near the surface in the polar zones submerge in tropical regions in order to 

 remain at the same temperature and salinity. Certain fishes of arctic coasts are found again 

 in the temperate zone at about 110 fathoms, the depth of the edge of the continental shelf. 

 .\t the equator they must be sought at great depths while in southern seas they reappear 

 at higher levels. Their ililTcring habitats are thus entirely circumscribed by the curves of 

 the isotherms. 



Other isotherms mark the journeys of migratory species, reproducing in far-away regions. 

 These plunge into the depths, constantly following the edge of the basin formed by the waters 

 suitable for them. In the tropics their sf)awning area is fixed by the lowermost point of this 

 edge. Through such vital cause and elTect, biological laws peculiar to marine species, their 

 feeding and reproduction, are presi<led over by other even stronger laws. These are cosmic 

 in order and hold I he oceans in swav. 



Surroundings and colours. 



Fishes do not all lead I he same kind of life. .More often tiiati not each species of a certain 

 family leads an individual life in its particular environment — an environment in which it meets 

 other forms of very dilTerenl origins. In company with these if is subjected to the same changes 

 of temperature and salinity, seeks the same kind of food and defends itself against the same 

 enemies. Within this community, which has existed for centuries in the same " climate ". 

 remarkable resemblances have developed, due to similarity of habits. Such animals can be 

 recognised as belonging to the same ecological type, this being apparent in external features 

 of form and colour. Thcic are also bidjotrical affinities. 



The shore-dii'i'lling type. Fishes of this type live in the tidal zone ami exlctid downwards 

 to 20 or 30 fal bonis - depths marking the lower limit of their shelters, the .seaweeds. In another 

 book, " Marine life of western European coasts", I have described the habits of the dwellers in 

 our coastal waters, but later on we shall have to conjure up the fishes of equatorial mangrove 

 swamps and of the lagoons of volcanic coral islands. Littoral fishes are not very sensitive 

 to heat or cold, for changes in the weather have a far-reaching elfect on the shallower waters. 

 Even if the winters are very cold or the sun burns fiercely, they rarely move far down among 

 the algae or on to sandy banks. Some species are euryhaline and venture into estuaries, but 

 most are confined to salt waters and cannot even tolerate sudden torrential downpours. Certain 

 species, however, have become adapted so as to live in brackish waters or in regions aroiind 

 the outflow of great tropical rivers. Nearly all coastal fishes display some form of camouflage. 

 Many bristle with spines, fleshy tags and other appendages, these blending with the disruptive 

 outlines of seaweeds. Their colour pattern matches that of marine plants, rocks, shingles and 

 sands, being in general brownish or greenish and overlaid with dull heavy marblings or very 



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