ii Transactions of the South African Philosophical Society. 



conformity to the surrounding conditions, but we do not so readily 

 realise, and I venture to say that even workers in this particular 

 line of investigation do not yet fully realise how intimately and 

 in what way marine life is co-ordinated with its environment, 

 more especially with its purely physical features. This is par- 

 ticularly the case with regard to the pelagic forms, or forms not 

 modified by the varying surroundings of shore life, in which such 

 factors as physical conformity of coast-line, nature of the bottom, 

 presence or absence of vegetable life, &c, play an important part, 

 but forms which are moulded according to the simple varying 

 elements in sea-water. These are, briefly, the comparatively slight 

 variations in the amount of salt contained in solution, and the 

 more marked variation in temperature. 



It is to this that I wish in the first place to direct your attention, 

 and in order to do so I will briefly review some of the more important 

 facts which have been ascertained with regard to the physical 

 features of the waters on the South African coast. 



You are aware that as a result of the unequal atmospheric 

 pressure there are two great currents of wind — the Trades blowing 

 in an oblique direction from north-east in the northern hemisphere, 

 and from the south-east in the southern hemisphere, and that these 

 give rise to a surface movement of the waters over which they pass 

 so that there is a general movement of the waters from the poles in 

 corresponding directions towards the equator where they meet' and 

 flow in a westward direction parallel to each other, forming the 

 great equatorial currents of the northern and southern hemispheres. 

 There are, however, two great continuous land barriers interposed 

 in the course of these currents, Europe, Asia, and Africa forming 

 one, North and South America forming the other. In addition 

 there is another barrier, with gaps, however, viz. : that formed by 

 discontinuous land masses in the meridian of Australia, the East 

 Indies, and China. This third barrier I mention separately for a 

 reason which shall appear later. 



The result of the interpolation of these land masses is that the 

 bodies of water thus set in motion and which must find an outlet to 

 maintain the general oceanic equilibrium are directed towards the 

 poles again. This is, of course, most clearly marked in the parts of 

 the Indian, Pacific, and the Atlantic Oceans situated in the southern 

 hemisphere, for there the movements of the waters are less inter- 

 rupted by land masses, and inasmuch as the Pacific is comparatively 

 shallow, and the movement of the water is there much interrupted 

 by islands and coral reefs, it is in the Indian and South Atlantic 

 Oceans that this phenomenon is best illustrated. 



