Moreover, there is the chance of
small changes being lost by intercrossing: our own cattle-breeders have
most carefully to select the parents, or else the favourable variety
soon disappears.
How then, seeing the power of stability which at least some forms are
found to exhibit--seeing too the enormous chances against the survival
of the particular specimens that begin to vary, and the further chances
of the loss of variety by intercrossing; how can we get the millions of
millions of years necessary to produce the present extreme divergence of
species? The fact is that the force of this objection is likely to be
undervalued, from the mere difficulty of bringing home to the mind the
immeasurable time really demanded by uncontrolled evolution.
Nor is the question of time left absolutely to be matter of belief or
speculation. For here and there in the geological records of the rocks,
we _have_ certain intermediate forms--or forms which we may fairly argue
to be such. But looking at the very considerable differences between the
earlier and the later of these forms--differences greater than those
which now separate well-defined species, it seems questionable whether
any of the divisions of Tertiary time, taking all the circumstances into
consideration, could be lengthened out sufficiently to accomplish the
change.
Pages:
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93