THE EVOLUTION OF PLANTS 89 



plant that existed was not very different from this, and 

 like it dwelt in water. Nor are we likely to be wrong 

 in thinking that the whole group of the seaweeds arose 

 by gradual development from such a form, perhaps 

 before any plants grew upon the surface of the land. 

 If we examine this group of seaweeds we find a 

 succession of forms which are step by step more 

 elaborate, and we see therein some justification for 

 thinking that development or evolution has proceeded 

 on similar lines. There are two points that may well 

 be insisted upon as a preliminary to thinking about 

 these changes: first, there is a predisposition in the 

 living substance to become more highly organised; 

 second, this organisation is brought about by the action, 

 direct or indirect, of the surroundings, the plant suc- 

 ceeding best which is in the most complete harmony 

 with them. We have already illustrated the second of 

 these points so far as the higher land plant is concerned. 



It is possible that the original ancestors of the sea- 

 weeds were even simpler than the one we have described, 

 not possessing any cell wall. If so, the exposure of the 

 living substance to the changes in the water and to 

 contact with obstacles in it no doubt led to the formation 

 of this cell wall for purposes of protection. 



The development of this membrane round it, how- 

 ever, introduced a difficulty in its relations with the 

 water, free access to which for so many reasons was very 

 necessary to its welfare. We can understand that the 

 development of the vacuole, with its little store of water, 

 was speedily effected to obviate this difficulty. 



Increase of size of the protoplast was no doubt one of 

 the. early features of the life of the plant. This in the 

 condition of a cell clothed by a membrane was only 

 possible up to a certain point without weakening the 

 membrane and so disturbing the protection it had 

 secured. Growth was consequently followed by divi- 



