254 SCIENCE PROGRESS . 



tropical species of the same genus has left its original marine 

 habit yet another step behind, since it occurs among grass on | 

 the top of high cliffs, or is found climbing low shrubs. Take * 

 the robber crabs which climb coconut-trees in the islands of 

 the Pacific. They present almost an exact analogy to the 

 Amphibia proper. The adults have become essentially terres- 

 trial animals — their gills have become modified and adapted 

 to breathing atmospheric oxygen — and they only return to 

 the water in order to reproduce their kind. Yet another ,j 

 very interesting form in this connection is a little tropical fish 

 called Periophthalmus, which has acquired the habit of sunning 

 itself on rocks at low tide, or of creeping about over sand-flats 

 by means of its pectoral fins. In relation with this habit 

 Periophthalmus has become exceedingly alert and watchful ; 

 the eyes are very prominent, and move about independently 

 as if on swivels. AH these examples give one the impression 

 that a second invasion of the land is being attempted directly 

 from the seashore, and would perhaps have met with more 

 success than it has done were the ground not already pre-empted 

 by the descendants of the prior invaders. Certainly, even this 

 partial success would scarcely have been possible were it not 

 for the action of the tides, which provide precisely the con- 

 ditions necessary. An animal starts a very short distance 

 above the low-water mark of spring-tides, where it has to face 

 desiccation only for an hour or so every fortnight. From there 

 some of its descendants are able to creep to a point where they 

 are uncovered a little more frequently and a little longer each 

 time, and so on by gradual stages to high-water mark of spring- 

 tides or above. The tides, in fact, by their daily movements, 

 and by their increase and decrease following the different 

 phases of the moon, make the shore zone an effective bridge 

 between land and sea ; another biological effect of their 

 action. 



With regard to the rhythmical effects of the tides, it is not 

 our intention to spend much time on this subject, as we have 

 already dealt with it in another place. ^ To put the matter 

 briefly, the tides impose a periodicity upon the behaviour of 

 shore forms, and this tends to become impressed upon the 

 animal and to persist for a time even when the latter is removed 

 from the tidal influence. This is well illustrated by the tiny 

 flat-worm Convoluta roscoffensis, which lives in great colonies 

 on certain flat, sandy beaches. At low tide the worms are at 

 the surface ; as the tide reaches them they begin to disappear. 

 The worms are thus constantly moving up and down at intervals 



^ Flattely, F. W., " Rhythm in Nature," Science Progress, Jan. 1920. 

 Cf. also Reynolds, W. E., '' The Cycles and Super-cycles of Nature," ibid, 

 Oct. 1920. 



