CHAPTER III
THE RELATIONS OF THE ABYSMAL ZONE AND THE ORIGIN OF ITS FAUNA
In the study of the geographical distribution of terrestrial animals one of the great difficulties met with is the impossibility of defining exactly the limits of the regions into which we divide the surface of the earth. In a general way we recognise that there is an Australian region, an Ethiopian region, &c.; but, when we come to discuss the exact position of the frontier lines that separate these regions from their neighbours, we find all kinds of difficulties to overcome and inconsistencies to meet.
For the sake of convenience it is useful to adopt certain arbitrary limits for these regions, notwithstanding these difficulties and inconsistencies, but we must recognise the fact that nature recognises no such limits, that every region overlaps its neighbours to a greater or less extent, and that there are many debateable grounds in the world where the fauna characteristic of one region is mixed with that characteristic of another.
But this difficulty in defining the exact limits of the terrestrial faunistic regions is even more pronounced in the case of the regions and zones of the marine fauna.
On the dry land we find mountain ranges, forests, deserts, and other barriers, that to a very considerable extent prevent the mixing of one fauna with another, but in the sea there are no barriers of anything like the same importance, but one fauna gradually merges into the neighbouring fauna according to the temperature, the pressure, the amount of light, the salinity of the water or the food supply. This then is one of the difficulties met with in the study of the geographical distribution of the marine fauna.
But there is another that leads to almost greater complications. In considering terrestrial life it is customary to refer only to regions of geographical, or perhaps it would be more correct to call it—superficial distribution. It would be quite possible, however, to subdivide the geographical areas into zones of elevation above the sea-level, not very clearly marked off from one another, it is true, but nevertheless each showing a number of characteristic features. This idea is expressed, for example, when we speak of the Alpine fauna, the Himalayan fauna, or the fauna of the great Andes.
In the study of the marine fauna and flora we must notice, it is the depth of the water, or in other words the depression of the habitats below the sea-level, that forms the most important consideration. Geographical sub-regions may be recognised and defined with a certain amount of accuracy, especially in the case of the fauna of the shallow waters, but by far the most important changes in the general characters of the fauna are found when we pass from one ‘zone’ of depression to another. Thus in describing any particular marine fauna we should mention first of all its zone or sub-zone of depression and then its geographical region and sub-region. For example, we may speak of the fauna of the pelagic zone of the British sub-region of the European region, or the fauna of the abysmal zone of the Northern sub-region of the Atlantic region.
We can recognise three primary zones of the marine fauna which we may call the ‘Pelagic,’ the ‘Neritic,’ and the ‘Abysmal’ zones.
The Pelagic zone includes the superficial waters of all seas extending from the surface to a depth which cannot at present be very accurately determined, but is probably the same as the limit of the influence of direct sunlight.
The animals of this zone are frequently characterised by a general transparency of their tissues, a white or sea water (i.e. blue or green) colour, an organisation capable of prolonged swimming or floating movement, and by giving birth to floating eggs which hatch out transparent larvæ or embryos.