The distribution of animals

Scientists are convinced that life came from the sea and from there the living organisms conquered, through the necessary evolutionary stages, both the Earth and the internal freshwaters. These steps, moving from the sea to the other environments, occurred in very ancient times, when the living forms were poorly developed and poorly specialised.
Later on, the living beings, even though spreading all around, found impassable boundaries that confined them to certain regions. These boundaries were mountain ridges, deserts, seas, rivers, temperatures, available water, the presence of oxygen in the water, geological events (continental drift, rising of the sea).
Animals can be divided on the basis of an ecological criterion (i.e. in relation to the environments in which they live) into marine, freshwater and terrestrial organisms. In between the first two there should be the organisms that live in brackish waters, waters of passage between salty waters and freshwaters. A separate category of terrestrial animals are those that live in tunnels, caves and in the cracks of the ground, animals that altogether compose the “hypogean fauna”.

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