DEGENERA TION. 19 



to a submerged forest-tree would lead one to expect ; 

 at the same time it is more difficult than those 

 who have boldly attempted it appear to believe. We 

 have one great help in the carefully worked out 

 systematic classification of animals and plants accord- 

 ing to their structure. We are justified in assuming 

 as a general law that animals or plants of like 

 structure have descended from common ancestors — 

 that is to say, that the same kind of organisation 

 (especially where a number of elaborate details of 

 structure are involved) has not been tivice produced 

 by natural selection. Thus we are entitled to con- 

 clude that all the animals which have a backbone 

 and pharyngeal gill-slits combined — the Vertebrates, 

 as we call them — have descended from a common 

 parent ; that all the animals with a muscular foot-like 

 belly and lateral gill filaments, the Molluscs, have also 

 had a common parent, and so. on. 



A classification according to structure goes then a 

 long way towards mapping out the main lines of the 

 family-tree of organisms. We are further assisted in 

 the task by the fossil remains of extinct organisms 

 which sometimes give to us the actual ancestors of 

 forms now living. But the most remarkable aid to 



C 2 



