1896.] *^^ [Bailey. 



eracy is unequivocally seen in certain restricted groups where the loss of 

 characters can be traced directly to adaptive clianges, as in the loss of 

 limbs in the serpents. Retarded evolution expresses the development of 

 the plant world better than the above terms, but even this is erroneous 

 because plant types exhibit quite as complete an adaptation to an enor- 

 mous variety of conditions as animals do, and there has been rapid prog- 

 ress towards specialization of structure. As a matter of fact, the vege- 

 table world does not exhibit, as a whole, any backward step, any loss of 

 characters once gained, nor any stationary 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 suigenetic. Or, in other 

 words, it is centrogenetic 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 genera- 

 tion develops sexual functions, and the product of the sexual union is an 

 asexual generation, and this, in turn, gives rise to another sexual gen- 

 eration like the first. In the lowest sex-plants, as the algse, 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 structure of the gametophyte, and is recog- 

 nizable as a separate structure only by students of special training. In 

 the fungi, which are probably of catagenetic evolution, alternation of 

 generations is very imperfect or wanting. In the true mosses, the 

 gametophyte is still the conspicuous part of the plant structure. It com- 

 prises all that part of the moss which the casual observer recognizes as 

 "the plant." The sporophytic generation is still attached to the per- 

 sistent gametophyte, and it is the capsule with its stem and appendages. 

 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 "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 

 the plant, tree or bush, as we see it. The gametophytic generation 

 is associated with the act of fertilization, the male prothallus or gameto- 

 phyte developing from the pollen grain and soon perishing, and the 

 female prothallus or gametophyte developing in the ovule and either soon 

 perishing or persisting in the form of the albumen of the seed. The great 

 development of the sporophyte in later time is no doubt a consequence of 

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

 has come the perfection of the centrogenic form. 



2. The Origin of 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 subjects of the 



