ALTERNATION OF GENERATIONS 483 



seems to be that doubling and reduction of chromosomes should regularly 

 succeed one another. It is otherwise, however, in organisms which grow 

 under less uniform conditions. In them the alternation of the two somatic 

 phases becomes more definitely stamped by difference of form, as it is seen to 

 be in the land-living Archegoniatae. 



The Land Habit. 



No circumstance of Life has more profoundly affected Organic 

 Evolution than the progression from water to land. The further the 

 comparative study of the simpler living beings is carried, the more 

 tlae conclusion is confirmed that the birth-place of Animals and 

 Plants was in the water. The comparative study of the Higher 

 Animals and Plants demonstrates as fully that it is in sub-aerial 

 conditions that both have reached their highest development. Accord- 

 ingly it becomes a question of supreme interest what are the effects 

 impressed upon the organism by a Land-habit in place of its original 

 aquatic surroundings. Certain factors stand out clearly. First, 

 the mechanical requirement for support and maintenance of form. 

 The body, whether of Animal or Plant, is nearly of the specific gravity 

 of water. When immersed it is buoyed up, and provided the water 

 be not itself in violent motion there is little demand on the organism 

 for mechanical resistance. It is different, however, with sub-aerial 

 organisms, whicli require not only to support their own weight 

 in tlie lighter medium of air, but they must also maintain 

 their form under the stress of winds, and other strains incident 

 to life on land. The larger the organism the higher still will be 

 the ratio of the demand. Such questions of mechanical strength 

 have been taken up for Plants in Chapter IX., and need not 

 be discussed again here. A second factor is the need for protection 

 of the protoplast against loss of water by evaporation. The evidence 

 of its importance is seen in the very general presence of a cuticularised 

 wall covering all the exposed surfaces, as well as in the simple fact 

 that no Land-living Plant sheds its ova from the parent, as so many 

 Algae do. 'A third factor is the need for internal aeration of the 

 tissues wherever they assume large bull:. Another which has contri- 

 buted to the moulding of Plant-Organisms living on land is the require- 

 ment of a large surface of exposure to light and air for Photo-Synthesis. 

 Such factors as these must be considered in their effect on the Evolu- 

 tion of Organisms shovs'ing sexuality and alternation, as they adapted 

 themselves to a Land-habit. 



It is not known from what source the Land-Vegetation sprang. 



