18 THE SURVIVAL OB^ THE UNLIKE. I. 



table world does not exhibit, as a whole, any backward 

 step, any loss of character once gained, nor any station- 

 ary or retarded periods ; but its progress has been widely- 

 unlike that of the animal world, and it has not reached 

 the heights which that line of ascent has attained. The 

 plant phylum cannot be said to be catagenetic, but it is 

 sui generis ; or, in other words, it is centrogeuetic, as 

 distinguished from dipleurogenetic. 



The hearer should be reminded, at this point, of the 

 curious alternation of generations which has come about 

 in the plant world. One generation performs sexual 

 functions, and the product of the sexual union is an 

 asexual generation, and this, in turn, gives rise to an- 

 other sexual generation like the first. In the low scx- 

 plants, as in some of the algSB, the sexual generation — 

 or the gametophyte, as it is called — generally comprises 

 the entire plant body, and the asexual generation — or 

 sporophyte — develops as a part of the fructifying struc- 

 ture of the gametophyte, and is recognizable as a sepa- 

 rate structure only by students of special training. In 

 the true mosses, the gametophyte is still the conspicuous 

 part of the plant structure. It comprises all that part of 

 the moss which the casual observer recognizes as "the 

 plant." The sporophytic generation is still attached to 

 the persistent gametophyte, and it is the capsule, with its 

 stem and appendages. In the ferns, however, the gamet- 

 ophytic stage is of short duration. It is the incon- 

 spicuous prothallus, which follows germination of the 

 spore. Therefrom originates "the fern," all of which 

 is sporophytic, and the gametophyte perishes. With 

 the evolution of the flowering plants, the gametophyte 

 becomes still more rudimentary, whilst the sporophyte 

 is developed into the plant, tree or shrub, as we see it. 



