NOVEMBER, 1905. OECOLOGICAL FEATURES OF EVOLUTION. 
179 
The first is in common with the vertebrates but the last distin- 
guishes it from its nearest ally. The variations permitting the 
use of new groups of contact thus departs farther and farther 
from the original structure and the environment appropriate to 
it. Parasitism and degeneration are just as definitely a following 
of this law as the attainment of a highly complex structure. 
The result may be a peripheral development with strengthen- 
ing of the new and gradual loss of the old, as a plant which 
propagates by suckers may be strong at the outer ends but the 
parent center dead. The classic case of the embryonic gill slits 
in the mammals illustrates well the final rudimentation of a once 
fundamental feature while the importance of the mammary glands 
is an equally good example of the strengthening of the new. 
The appearance of fishes, amphibians, reptiles, birds and 
mammals in order in geological time is no argument for contin- 
uous improvement, their inception and flower mark the conquer- 
ing of a new group of contacts so that almost every variety pro- 
duced found a hospitable sum of contacts awaiting: it. So lone 
as the fishes produced varieties fitted only to breathe in water so 
long was the possible hospitality of the contacts limited but when 
a variety was produced which could also breathe the air the 
possible group of contacts was increased so enormously that al- 
most every variable found a group of contacts hospitable to itself. 
This statement is not in conflict with the previous one that that 
environmental hospitality would have a tendency to keep down 
the establishment of new forms ; that would be true where the pos- 
sibilities of the environment were small but when a whole new 
realm is conquered, as the air in the case of the first air- 
breathers, the statement must be reversed until the new region 
is well filled. 
As to the series fish, amphibian, reptile, bird and mammal in- 
dicating improvement it would hardly have been suggested had 
our brains developed in the body of a fish or a parasitic worm, 
even as now we fail to appreciate the delicate adjustments or the 
organism of the serpent or the lizard. 
An interesting application of the theory arises in the develop- 
ment of the early vertebrates. The first of the air breathers 
quickly became entirely terrestrial in habit and only later returned 
to the sea under stress of environmental hostility on the land. 
Geology teaches that the Carboniferous age, in which the air- 
breathers first appeared in any prominence, was a time of great 
transgression of the sea, or great humidity and luxuriance of 
vegetation ; but at the end of the Carboniferous the sea retired 
