Of course
the "new" species is not new in the sense that its ancestors
appeared later on the globe's surface than those of any old
species tottering to extinction. Phylogenetically, each animal now
living must necessarily trace its ancestral descent back through
countless generations, through aeons of time, to the early stages of
the appearance of life on the globe. All that we mean by a "new"
species is that from some cause, or set of causes, one of these
ancestral stems slowly or suddenly develops into a form unlike any
that has preceded it; so that while in one form of life the ancestral
type is continuously repeated and the old species continues to exist,
in another form of life there is a deviation from the ancestral type
and a new species appears.
Similarly, "extinction of species" is a term which has two entirely
different meanings. The type may become extinct by dying out and
leaving no descendants. Or it may die out because as the generations
go by there is change, slow or swift, until a new form is produced.
Thus in one case the line of life comes to an end. In the other case
it changes into something different. The huge titanothere, and the
small three-toed horse, both existed at what may roughly be called the
same period of the world's history, back in the middle of the
mammalian age.
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