their origin. The combined investigations of another 20 years may,
perhaps, enable naturalists to say whether the modifying causes and the
selective power, which Mr. Darwin has satisfactorily shown to exist in
nature, are competent to produce all the effects he ascribes to them,
or whether, on the other hand, he has been led to over-estimate the
value of his principle of natural selection, as greatly as Lamarck
overestimated his vera causa of modification by exercise.

But there is, at all events, one advantage possessed by the more recent
writer over his predecessor. Mr. Darwin abhors mere speculation as
nature abhors a vacuum. He is as greedy of cases and precedents as any
constitutional lawyer, and all the principles he lays down are capable
of being brought to the test of observation and experiment. The path
he bids us follow professes to be, not a mere airy track, fabricated of
ideal cobwebs, but a solid and broad bridge of facts. If it be so, it
will carry us safely over many a chasm in our knowledge, and lead us to
a region free from the snares of those fascinating but barren Virgins,
the Final Causes, against whom a high authority has so justly warned us.
"My sons, dig in the vineyard," were the last words of the old man in
the fable; and, though the sons found no treasure, they made their
fortunes by the grapes.