26 DEGENERA TION. 



relationships of all existing forms of animals, natu- 

 ralists have hitherto assumed that the process of 

 natural selection and survival of the fittest has in- 

 variably acted so as either to improve and elaborate 

 the structure of all the organisms subject to it, or 

 else has left them unchanged, exactly fitted to their 

 conditions, maintained as it were in a state of balance. 

 It has been held that there have been some six or 

 seven great lines of descent — main branches of the 

 pedigree — such as that of the Vertebrates, that of 

 the Molluscs, that of the Insects, that of the Starfish, 

 and so on ; and that along each of these lines there has 

 been always and continuously a progress — a change 

 in the direction of greater elaboration. 



Each of these great branches of the family-tree is 

 held to be independent — they all branch off nearly 

 simultaneously from the main trunk like the leading 

 branches of an oak. The animal forms constituting 

 the series in each of these branches are supposed to 

 gradually increase in elaboration of structure as Vv'e 

 pass upwards from the main trunk of origin and 

 climb further and further towards the youngest, 

 most recent twigs. New organs have, it is supposed, 

 been gradually developed in each series, giving their 



