CRYPTOGAMS 359 



Roughly speaking, it may be said : (1) that Thallophytes 

 are predominantly aquatic ; (2) Archegoniates (Bryophytes 

 and Pteridophytes), amphibious; (3) Spermophytes, terres- 

 trial; (4) that the seed habit is a response to terrestrial 

 conditions; and (5) that the increased development of the 

 sporophyte was a necessary adaptation to meet those condi- 

 tions. 



IX. THE COURSE OF PLANT EVOLUTION 



417. Plant genealogy. — It has been shown by a study of 

 existing forms of plant life that there is no hard and fast 

 line of division anywhere between the different groups, but 

 that they are all connected by ties of kinship more or less 

 defined, according to their distance from a common ancestral 

 stock. The geological record points to the same conclusion, 

 and our classification of them into families, orders, and spe- 

 cies is merely a very imperfect genealogical table of their 

 supposed pedigrees. This does not mean, however, that we 

 can assert positively that such and such a species is derived 

 from such or such another, but that both are descended from 

 some common intermediate form more or less remote. While 

 we have reason to believe that the flowering plants are de- 

 rived through pteridophyte and bryophyte types from some 

 of the green algae, no direct connection has ever been traced 

 between any particular kind of flowering plant and any par- 

 ticular kind of alga, — or between a liverwort and an alga, 

 for that matter, — and probably never will be, because the in- 

 termediate forms die out, or pass on by variation into other 

 lines of development. But while this is true, all the evidence 

 we possess does go to show that, since the beginning of life 

 on the globe, there has been a general progressive evolution 

 from lower and simpler to higher and more complex forms. 



418. Retrogressive evolution. — AVhile the general course 

 of evolution has been upward and onward, the movement has 

 not always followed a straight line, but, like a mountain road, 



