PROTECTION. 17 



each component plant retaining both its physiological and 

 morphological individuality, and the explanation of this 

 colony-forming idea that naturally suggests itself is — resist- 

 ance to drought ; on the principle that a given amount of 

 matter in one piece, especially when surrounded by a 

 substance very retentive of moisture, takes a longer time to 

 become dry than an equal amount of the same kind of 

 matter broken up into numerous pieces. The correctness 

 of this view is supported by the fact that the colony-forming 

 species have a much wider range than their non-colony- 

 forming relations, owing to being able to survive a wider 

 range of surroundings, being equally at home when com- 

 pletely submerged, or in swampy places exposed to the air. 

 The amount of difference between the two last-named 

 habitats may not at first sight appear to be very pronounced, 

 but it is a step in the right direction, and illustrates the 

 slow process of evolution. This difference of habit, or 

 relative faculty on the part of living organisms to adapt 

 themselves to circumstances is often very marked in different 

 species of the same genus. Conferva tortuosa forms fleecy 

 tufts on rocks which retain a sufficient quantity of water to 

 prevent dessication during low tide, thus enabling the plant 

 to continue its work under varied conditions, a circumstance 

 in favour of life, whereas Conferva melagonium consists of 

 isolated filaments, hence never flourishes above low-water 

 mark, and consequently is confined to one set of conditions. 

 The former has a wide distribution, the latter is not common 

 anywhere. Some of the large brown seaweeds store up a 

 certain amount of water in their tissues which enaWes them at 

 least to resist the injurious effects of dry air on tfieir tissues 

 during exposure between tides, possibly it may enable them 

 to do more than this. In spite of the progress made by 



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