There is no more difficulty in understanding how the branched
spines of some ancient Echinoderm, which served as a defence, became
developed through natural selection into tridactyle pedicellariae, than in
understanding the development of the pincers of crustaceans, through
slight, serviceable modifications in the ultimate and penultimate segments
of a limb, which was at first used solely for locomotion. In the
avicularia and vibracula of the Polyzoa we have organs widely different in
appearance developed from the same source; and with the vibracula we can
understand how the successive gradations might have been of service. With
the pollinia of orchids, the threads which originally served to tie
together the pollen-grains, can be traced cohering into caudicles; and the
steps can likewise be followed by which viscid matter, such as that
secreted by the stigmas of ordinary flowers, and still subserving nearly
but not quite the same purpose, became attached to the free ends of the
caudicles--all these gradations being of manifest benefit to the plants in
question. With respect to climbing plants, I need not repeat what has been
so lately said.
It has often been asked, if natural selection be so potent, why has not
this or that structure been gained by certain species, to which it would
apparently have been advantageous? But it is unreasonable to expect a
precise answer to such questions, considering our ignorance of the past
history of each species, and of the conditions which at the present day
determine its numbers and range.
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