12 THE BIOLOGY OF THE SEA-SHORE 



give a readily recognised, distinctive appearance or " facies " 

 to that particular part of the shore. In proceeding from high 

 to low water mark a succession of these will be crossed. 

 If the nature of the substratum remains unaltered the 

 differences in the composition of the fauna are chiefly 

 determined by the period of exposure the individual forms 

 are capable of surviving. Moreover, so regular is the 

 tidal rhythm that some forms have come to require a certain 

 definite period of exposure and will perish if kept con- 

 tinually submerged. In passing, therefore, from high to 

 low water mark, while new forms are continually coming 

 in, others will be found to drop out. Frequently, however, 

 a shore is not of a uniform type, in which case a particular 

 zone, when followed for a short distance, will be interrupted 

 owing to the interpolation of a dijfferent physical type 

 (sand may succeed rock, etc.), but will reappear with its 

 characteristic plant and animal association when the same 

 type of shore is again met with. It is impossible to draw 

 up a classification of shores which will fit all cases, but that 

 of Pruvot reproduced opposite is of fairly general application 

 and will serve as a working guide, and for the further 

 assistance of the student we have included in the 

 Appendix a chart of a typical piece of shore of mixed 

 physical character with a list of the common forms occurring 

 there. 



In addition to the linear zoning of shore forms, the vertical 

 aspect also requires consideration. It is only in the case 

 of completely flat shores that submergence proceeds uni- 

 formly, and on certain shores the height of the rocks or 

 boulders becomes of great importance. Suppose, for 

 instance, the vertical range of the tides to be ten feet, the 

 summit of a rock of this height situated in the Laminarian 

 zone will frequently carry an association in all respects 

 identical with that occurring at high-tide mark. This 

 will be seen in the chart referred to, where the succession 

 of algae clothing the sides of the rocky reef, as one passes 

 from top to bottom in the region of low-tide mark, exactly 

 repeats the linear arrangement found on passing from high 



