SOME PRINCIPLES OF PLANT EVOLUTION 457 



its nutrition and growth of mass, finally producing spores again 

 which complete its entire life cycle. This is evolution of the 

 individual* There is change and progression, in a natural 

 way, i.e., according to natural laws. 



643. There are, however, different steps in the individual 

 evolution of the fern, some of which are quite clearly marked, 

 so that we can recognize them. We can also recognize that they 

 have a resemblance to lower forms of plant life. The asexual 

 spore recalls the single-celled plants, the protonema recalls the 

 thread-like algae, and the prothallium recalls green tissue plates 

 or membranous masses of cells. So it is with sexual organs and 

 the very early stages of the embryo. We see then that there are 

 plants living to-day which in their mature condition represent or 

 picture, as it were, some of the different stages in the individual 

 evolution of the fern plant. Some of them have made little 

 progress or change from very simple one-celled plants. Others 

 progressed for a time and then stood still, or only further differ- 

 entiation took place. They have lagged behind, and thus never 

 reached the high state of development shown in the fern plant. 

 The seed plants show a still further progress and 'a higher state 

 of development. 



644. The evolution, or life history, of the individuals in the 

 different groups of plants, if read aright, teaches us that the 

 higher groups have been built on the experience in the evolutions or 

 life histories of some of the lower groups. There has been change 

 and progression according to natural laws from the low unicellular 

 plants, upward, resulting in the seed plants, the dominant vege- 

 tation on the earth at the present time. This is the evolution f of 

 higher groups from the lower groups along lines which often can 

 be traced, as regards the more general features. So it is with 

 the animal kingdom; the higher forms have been developed 

 from the lower forms, as a study of the life histories or evolu- 



* Usually called Ontogenetic evolution (&v, 6i>ros = being ; 7^e<rt$ = 

 generation or development). 



f The tracing of lines of evolution from lower groups to higher groups 

 is called Phytogeny. 



