THE INTRODUCTION OF NEW 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 



