1 3 DEGENERA TION. 



forms produced) to larger and larger branches, until 

 the common trunk representing the original ancestor 

 would be reached. Our actual means of observing the 

 genealogical affinities of different kinds of animals and 

 plants may be understood by a further use of the meta- 

 phor 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 mudd}' 

 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 



