456 FACTORS OF ORGANIC EVOLUTION. 



ary or retarded periods; but its progress has been widely unlike tliat 

 of tlie animal world and it lias not reached the heights which that line 

 of ascent has attained. The plant phylum can not be said to be cata- 

 genetic, but suigenetic. Or, in other words, it is ceutrogenetic as 

 distinguished from dipleurogenetic. 



The hearer should be reminded, at this point, of the curious alterna- 

 tion of generations which has come about in the plant world. One 

 generation develops sexual functions, and the product of the sexual 

 union is an asexual generation, and this, in turn, gives rise to another 

 sexual generation like the first. In the lowest sex plants, as the 

 algse, the sexual generation — or the gametophyte, as it is called — gen- 

 erally comprises the entire plant body, and the asexual generation or 

 sporophyte develops as a part of the fructifying structure of the 

 gametophyte, and is recognizable as a separate structure only by 

 students of special training. In the fungi, which are probably of cat- 

 agenetic evolution, alternation of generations is very imperfect or 

 wanting. In the true mosses the gametophyte is still the conspicuous 

 part of the jilant structure. It comprises all that part of the moss which 

 the casual observer recognizes as "the plant." The sporophytic gener- 

 ation is still attached to the persistent gametophyte, and it is the 

 capsule with its stem and ajjpendages. In the ferns, however, the 

 gametophytic stage is of short duration. It is the inconspicuous 

 prothallus, which follows the germination of the spore. Therefrom 

 originates "tlie fern," all of which is sporophytic, and the gametophyte 

 perishes. With the evolution of the flowering i)lants the gametophyte 

 becomes still more rudimentary, while the sporophyte is the plant, 

 tree, or bush as we see it. The gametophytic generation is associated 

 with the act of fertilization, the male prothallus or gametophyte devel- 

 oping from the x)ollen grain and soon perishing, and the female pro- 

 thallus or gametophyte developing in the ovule and either soon 

 T)erishing or persisting in the form of the albumen of the seed. The 

 great development of the sporophyte in later time is no doubt a conse- 

 quence of the necessity of assuming a terrestrial life; and with this 

 develoioment has come the perfection of the centrogenic form. 



2. THE ORIGIN OP DIFFERENCES. 



The causes which have contributed to the origin of the' differences 

 which we see in the organic creation have been and still are the sub- 

 jects of the most violent controversy. Those x3ersons who conceive 

 these differences to have come into existence full formed, as they exist 

 at the i)resent time, are those who believe in the dogma of special crea- 

 tions, and they usually add to the doctrine a belief in design in nature. 

 This doctrine of special creation receives its strongest support when 

 persons contrast individual objects in nature. Certainly nothing can 

 seem more unlike in very fundamental character than an insect and- an 

 elephant, a starfish and a potato, a man and an oak tree. The 



