370 ON THE FAUNA AND BOTTOM-DEPOSITS NEAR THE 30-FM. LINE 



Of these four variable factors the second, the presence of soluVjle 

 substances derived from tlie drainage of the land, has never, so far as 

 I am aware, received any attention, although in many localities, 

 especially in enclosed estuaries, it is probably of considerable import- 

 ance in influencing the distribution of animal and plant life. 



The incursion of fresh-water is a factor which is liable to great 

 variation. At times of exceptional ilood the influence of a river will 

 be felt at a much greater distance from shore than at other times. In 

 considering the physical conditions of any particular ground it is 

 necessary to bear this in mind, and to know whether it is or is not 

 subject to such periodic incursions of water of low density at regular 

 or irregular intervals. It is a well-known fact that marine animals are 

 liable to be injured by a sudden change in the density of the water of a 

 much smaller amount than that to whicli they can quite well become 

 accustomed, provided the change takes place gradually. 



2. The Nature of the Bottom-deposit. Since the time of the earliest 

 marine naturalists it has been fully recognised that one of the principal 

 factors in determining the distribution of marine animals and plants is 

 the nature of the bottom-deposit or substratum upon which they live. 



The nature of the substratum varies both as to the kind of material 

 of which it is composed and as to the texture of that material. The 

 kind of material forming the bottom-deposit may be either inorganic, 

 when it varies with the geological formation of the particular locality, 

 or it may be organic, consisting of the skeletal remains of marine 

 organisms, in which case it varies according to the fauna of the locality. 

 Deposits composed almost entirely of organic remains have been termed 

 neritic deposits by Herdmau. The texture of the bottom-deposit varies 

 from solid rock or large stones to gravel, sand, and the finest mud. 

 Amongst marine animals and plants, especially amongst the former, a 

 great variety of adaptations are found for fixing, for boring, for burrow- 

 ing, and for locomotion, specially suited to the particular kind of bottom 

 upon which the organism generally lives. In very few cases, however, 

 have the different kinds of bottom-deposit, upon which a particular 

 species can live, been accurately ascertained, and one of the results 

 of such investigations as that detailed in the present report is to obtain 

 additional evidence on such questions. 



3. The Movements of the Water, (a) Wave action. The important 

 infiuence exerted by the action of the waves on animals and plants 

 living between tide-marks, or in water of only a few fathoms depth, 

 has always been recognised by naturalists, and many of the adaptations 

 of structure and habit, by means of which this action is combated, 

 have been pointed out. It was not, however, until the appearance of 

 Hunt's paper (No. 54) on the influence of wave currents that the atteu- 



