As knowledge increases our wisdom is often turned
into foolishness, and many of the phenomena of evolution which seemed
clearly explicable to the learned master of science who founded these
lectures, to us nowadays seem far less satisfactorily explained. The
scientific men of most note now differ widely in their estimates of
the relative parts played in evolution by natural selection, by
mutation, by the inheritance of acquired characteristics; and we study
their writings with a growing impression that there are forces at work
which our blinded eyes wholly fail to apprehend; and where this is the
case the part of wisdom is to say that we believe we have such and
such partial explanations, but that we are not warranted in saying
that we have the whole explanation. In tracing the history of the
development of faunal life during this period, the age of mammals,
there are some facts which are clearly established, some great and
sweeping changes for which we can with certainty ascribe reasons.
There are other facts as to which we grope in the dark, and vast
changes, vast catastrophes, of which we can give no adequate
explanation.
Before illustrating these types, let us settle one or two matters of
terminology. In the changes, the development and extinction, of species
we must remember that such expressions as "a new species," or as "a
species becoming extinct," are each commonly and indiscriminately
used to express totally different and opposite meanings.
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