12 : NATURAL SELECTION I 
been from a lower to a higher degree of organisation. The 
admitted facts seem to show that there has been a general, 
but not a detailed progression. Mollusca and Radiata existed 
before Vertebrata, and the progression from Fishes to Reptiles 
and Mammalia, and also from the lower mammals to the 
higher, is indisputable. On the other hand, it is said that 
the Mollusca and Radiata of the very earliest periods were 
more highly organised than the great mass of those now 
existing, and that the very first fishes that have been dis- 
covered are by no means the lowest organised of the class. 
Now it is believed the present hypothesis will harmonise 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 pro- 
gression in the scale of organisation 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 bestmode 
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 Mollusca) has attained to 
a great richness of species and a high organisation. 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 successively created, having for their antitypes the 
same lower organised species which had served as the anti- 
types 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 organisation given to it, and becomes the repre- 
sentative group of the former one in another geological form- 
ation. It may, however, happen, that though later in time, 
the new series of species may never attain to so high a degree 
of organisation as those preceding it, but in its turn become 
extinct, and give place to yet another modification from the 
same root, which may be of higher or lower organisation, 
more or less numerous in species, and more or less varied in 
form and structure, than either of those which preceded it 
