16 DEGENERATION : I 



kinds of animals and plants may be understood by a 

 further use of the metaphor of a genealogical tree in 

 shape like an elm or an oak. Suppose the genealogical 

 tree completely written out — a perfect record — to be 

 sunk in muddy water so that only its topmost branches 

 and twigs are here and there visible — then you have a 

 fair notion of the present condition of the great family 

 of organisms. Only the topmost twigs remain visible, 

 the rest of the great family-tree of living beings is 

 hidden from view, submerged beneath the muddy 

 waters of time. Naturalists have, however, undertaken 

 to reconstruct this great genealogical tree. It is a 

 main object now in the study both of zoology and of 

 botany to find out what are the cousinships, what the 

 exact genetic relationships of all the various species 

 of plants and animals, and so to show, even to the 

 minutest detail, in what particular ways physico- 

 chemical causes have brought about and modified the 

 forms of living things. 



The task is not quite so difficult as the comparison 

 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 according to their 

 structure. We are justified in assuming as a general 

 law that animals or plants of like structure have de- 

 scended from common ancestors — that is to say, that 

 the same kind of organisation (especially where a num- 

 ber of elaborate details of structure are involved) has 

 not been twice produced by natural selection. Thus 



