130 
NATURAL SELECTION. 
Chap. IV. 
sizes may represent tliose whole orders, families, and 
genera which have now no living representatives, and 
which are known to us only from having been found in 
a fossil state. As we here and there see a thin strag¬ 
gling branch springing from a fork low down in a tree, 
and which by some chance has been favoured and is 
still alive on its summit, so we occasionally see an 
animal like the Ornithorhynchus or Lepidosiren, which 
in some small degree connects by its affinities two large 
branches of life, and which has apparently been saved 
from fatal competition by having inhabited a protected 
station. As buds give rise by growth to fresh buds, and 
these, if vigorous, branch out and overtop on all sides 
many a feebler branch, so by generation I believe it 
has been with the great Tree of Life, which fills with 
its dead and broken branches the crust of the earth, 
and covers the surface with its ever branching and 
beautiful ramifications. 
