the Introduction of New Species. 191 



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 ^lollusca and Kadiata 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 organized of 

 the class. Now it is believed the present hypothesis will har- 

 monize with all these facts, and in a gi-eat measure serve to 

 explain them; for though it may appear to some readers essen- 

 tially a theory of progression, it is in reality only one of gradual 

 change. It is, however, by no means dilficidt 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 geolo- 

 gical epoch any group (say a class of the Mollusca) has attained 

 to a great richness of species and a high organization, is'^ow let 

 this great branch of allied species, by geological mutations, be 

 completely or partially destroyed. Subscquentl}'^ 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 organiza- 

 tion given to it, and becomes the representative group of the 

 former one in another geological formation. It may, however, 

 happen, that though later in time, the new series of species 

 may never attain to so high a degree of organization 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 organization, more or less numerous in species, 

 and more or less varied in form and structure than either of 

 those which preceded it. Again, each of these groups may not 

 have become totally extinct, but may have left a few s])ecies, 

 the modified prototypes of which have existed in each succeeding 

 period, a faint memorial of their former grandeur and luxuriance. 

 Thus every case of apparent retrogression may be in reality a 

 progress, though an interrupted one : when some monarch of 

 the forest loses a limb, it may be replaced by a feeble and sickly 

 substitute. The foregoing remarks appear to apply to the case 

 of the iNIollusca, which, at a very early period, had reached a 

 high organization and agrcat development of forms and spe- 

 cies in the Testaceous Cephalopoda. In each succeeding age 



