XIII EVOLUTION 143 



chairs or tables. The words affinity, relationship, &c., as 

 applied to different species are, on the theory of creation, 

 purely metaphorical, and mean nothing more than that a 

 certain likeness or community of structure exists ; just as 

 we might say that an easy chair was more nearly related to a 

 kitchen chair than either of them to a three-legged stool. 



We see therefore that on the hypothesis of creation the 

 varying degrees of likeness and unlikeness between the 

 species receive no explanation, and that we get no absolute 

 criterion of classification : we may arrange our organisms, 

 as nearly as our knowledge allows, according to their resem- 

 blances and differences, but the relative importance of the 

 characters relied on becomes a purely subjective matter. 



According to the rival theory — that of Descent or Organic 

 Evolution — every species existing at the present day is 

 derived by a natural process of descent from some other 

 species which lived at a former period of the world's 

 history. If we could trace back from generation to gener- 

 ation the individuals of any existing species we should, on 

 this hypothesis, find their characters gradually change, until 

 finally a period was reached at which the differences were so 

 considerable as to necessitate the placing of the ancestral 

 forms in a different species from their descendants at the 

 present day. And in the same way if we could trace back 

 the species of any one genus, we should find them gradually 

 approach one another in structure until they finally con- 

 verged in a single species, differing from those now existing 

 but standing to all in a true parental relation. 



Let us illustrate this by reference to Zoothamnium. As a 

 matter of fact we know nothing of the history of the genus, but 

 the comprehension of what is meant by the evolution of species 

 will be greatly facilitated by framing a working hypothesis. 



Suppose that at some distant period of the world's history 



