CHAPTER I 



SOME NATURAL LAWS 



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Life and laws in the ocean 



A salmon leaping in a limpid waterfall; a school of Liinny-fishes swimming rapidly in the 

 open sea ; the silvery glitter of a sardine shoal on the move ; the rainbow pink of surface cruising 

 mackerel; the spectral brilliance of gentle coral fishes; the slaty coat of a shark or swordfish 

 n the prowl — all these conjure up an idea of unboundeil freedom in the protean world of 

 nature. But this colourful splendour and this power and harmony of motion are merely appear- 

 ances masking an implacable determinism which rules all living beings. All are swayed by 

 definite laws appropriate to I lie surroundings in which they move, feed and multiply. Countless 

 species can only live by strictly conforming to these laws and keeping within those bounds 

 which have allowed them to evolve and develop. When cosmic or climatic changes rudely 

 upset this indispensable living space the animals die and extinct species swell the vast cemetery 

 of this fossil world — this record of the piling up of geological strata since life took its place 

 on the earth. 



Each year marine fishes underlakt; two .seasonal movements. The first is a breeding 

 migration, when individuals of the same kind gather together in an area essential for the spawn- 

 ing and fertilisation of the eggs. But the extent of this movement varies from one species to 

 another. In sedentary coastal fishes it can hardly be said to occur, although many other coastal 

 forms move downwards to the edge of the continental shelf. On the other hand, certain deep- 

 water fishes migrate upwards to shallower grounds. The great migratory species undertake 

 immense journeys that may nearly span the entire breadth of an ocean. 



Those species which leave the sea to spawn in fresh waters are said to be anadromous in 

 habit. Conversely, catadromous forms are those that move down the rivers and enter the 

 sea to spawn. Anadromous or catadromous movements can be complete, meaning that there 

 is a complete transference from salt to fresh water, or the converse. But they may only be 

 partial. A number of oceanic species at spawning time move upwards towards the coasts and 

 higher grounds, while others descend to depths beyond their normal habitat. 



These two ways of reproducing lead to very similar ends. Most anadromous fishes attach 

 their eggs to the bottom (demersal eggs), wdiile the catadromous forms lay buoyant, pelagic 

 eggs which float in the surface waters. In both instances the eggs profit from the light and 

 heat of the sun, so developing under favourable conditions. Whether laid in a thin layer of 

 water near river banks (anadromous habit) or whether shed in deeper water to float upwards 



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