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 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 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. 



