22 DEGENEEATION I I 



whicti I wish to draw the reader's attention. In 

 attempting to reconstruct the pedigree of the animal 

 kingdom and so to exhibit correctly the genetic 

 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 hekl 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 we 

 pass upwards from the main trunk of origin and 

 climb farther and farther towards the youngest, 

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

 been gradually developed in each series, giving their 

 possessors greater powers, enabling them to cope 

 more successfully with others in that struggle for 

 existence in virtue of which these new oroans 

 have been little by little called into being. At the 



