INTRODUCTION 9 



ately created, the word relationship was used in a purely 

 metaphorical sense, as there could of course be no real 

 relationship between two groups of animals having a totally 

 independent origin. But it was assumed that creation had 

 taken place according to a certain scheme in the Divine 

 Mind, and that the various species had their place in this 

 scheme like the bits of glass in a mosaic. The problem of 

 classification was thus to discover the place of each species 

 in the pattern of the unknown design. 



The point of view underwent a complete change when, 

 after the publication of Darwin's Origin of Species in 1859, 

 the Doctrine of Descent or of Organic Evolution came to 

 be generally accepted by biologists. A species is now 

 looked upon, not as an independent creation, but as having 

 been derived by a natural process of descent from some pre- 

 existing species, just as the various breeds of Domestic Fowl 

 are descended from the little Jungle-fowl of India. On this 

 view the resemblances between species referred to above 

 are actually matters of relationship, and species are truly 

 allied to one another in varying degrees, since they are de- 

 scended from a common ancestor. Thus a natural classifi- 

 cation becomes a genealogical tree, and the problem of 

 classification is the tracing of its branches. 



This, however, is a matter of extreme difficulty. Repre- 

 senting by a tree the whole of the animals which have ever 

 lived on the earth, those existing at the present day would 

 be figured by the topmost twigs, the trunk and main 

 branches representing extinct forms. Thus the task of ar- 

 ranging animals according to their relationships would be 

 an almost hopeless one but from two circumstances : one, 

 that remains of many extinct forms have been preserved : 

 the other, that the series of changes undergone by an 

 animal in its development from the egg often forms an 



