THE INTRODUCTION OF N#LW 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 arrange- 
ment 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 organization. Now 
let this great branch of allied species, by geologi- 
cal 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 
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 
