Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

THE INTRODUCTION OF th'W SPECIES. 15
it is said that the Mollusca and Radiata of the very
earliest periods were more highly organized than
the great mass of those now existing, and that the
very first fishes that have been discovered are by no
means the lowest organised of the class. Now it is
believed the present hypothesis will harmonize with
all these facts, and in a great measure serve to
explain them; for though it may appear to some
readers essentially a theory of progression, it is in
reality only one of gradual change. It is, however,
by no means difficult to show that a real progression
in the scale of organization is perfectly consistent
with all the appearances, and even with apparent
retrogression, should such occur.
Returning to the analogy of a branching tree, as
the best mode of representing the natural arrangement of species and their successive creation, let us
suppose that at an early geological epoch any group
(say a class of the M3ollusca) has attained to a great
richness of species and a high organization. Now
let this great branch of allied species, by geological mutations, be completely or partially destroyed.
Subsequently a new branch springs from the same
trunk, that is to say, new species are successivelyf
created, having for their antitypes the same lower
organized species which had served as the antitypes
for the former group, but which have survived the
modified conditions which destroyed it. This new
group being subject to these altered conditions, has
modifications of structure and organization given