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 alge, 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. — While the general course 
of evolution has been upward and onward, the movement has 
not always followed a straight line, but, like a mountain road, 
