48 FOOT-NOTES TO EVOLUTION. 
plants had their origin in pre-existing forms more or 
less different, we may presume this to have been true 
of man also. That it is true of man in fact we know, 
for not many thousands of years ago our ancestors in 
Europe were barbarians, cave dwellers, lake dwellers, 
and dwellers in hollow trees, using only the rude imple- 
ments they shaped from metal and flint. The origin of 
civilized man from barbarous man gives the clew to the 
origin of barbarous man from forms still less specialized. 
The question of the origin of man, though perhaps 
the most interesting problem in science, offers to the stu- 
dent of Nature peculiar difficulties. Materials for exact 
knowledge are few and prejudices are strong, and all 
tendencies favour an immediate decision on doubtful 
points, though the evidence be far from sufficient. Of 
not one man, nor monkey, nor bird, nor beast in half a 
million does a trace remain after a thousand years—not 
a bone, nora relic, nor a thought. Living on the sur- 
face, we crumble into dust; and the current phases of 
human life, a few centuries out of hundreds, are all of 
man’s history we surely know. Many links are missing 
still, and most of these we can never find. Our early 
ancestry we can best infer from our knowledge -of the 
embryonic history and mental development of the man 
of to-day. 
But if anything in science is certain, it is that homol- 
ogy is a fact, and that it has a meaning. Among us 
backboned animals, all structures, all 
functions, and all mental operations 
show distinct homologies. The essence 
of the development theory is this: Homology is the 
stamp of heredity. Homology means blood relation- 
ship. No other meaning of homology has ever been 
shown, nor is there the slightest evidence that any other 
interpretation is possible. Blood relationship implies a 
Meaning of 
homology. 
