RATE OF SPECIFIC CHANGE 51 



These considerations lead me to express a doubt whether 

 the biologists have been correct in looking for continuous 

 transformation of species. Judging by analogy we should 

 rather expect to find slight continuous changes occurring 

 during a long period of time, followed by a somewhat sudden 

 transformation into a new species, or by rapid extinction.' 



In order to clear up any doubts about the sense 

 in which the word * continuous ' is here employed, 

 the following footnote is appended to Sir George 

 Darwin's address : 



'If we may illustrate this graphically, I suggest that the 

 process of transformation may be represented by long lines 

 of gentle slope, followed by shorter lines of steeper slope. 

 The alternative is a continuous uniform slope of change. If 

 the former view is correct, it would explain why it should 

 not be easy to detect specific change in actual operation. 

 Some of my critics have erroneously thought that I advocate 

 specific change per saMum.' 



Biologists are doubtless prepared to agree with 

 the author's conclusions. Indeed, there is no 

 reason for the belief that they have ever looked 

 for a continuous and uniform rate of specific 

 change, so clear has been the evidence afforded 

 by the persistence of ancestral forms in certain 

 areas as compared with their modification or 

 extinction in others (see pp. 46, 47). 



THE FIFTIETH ANNIVERSARY OF THE ORIGIN 

 OF SPECIES A RETROSPECT 



That the Origin of Species, of which Darwin said 

 ' It is no doubt the chief work of my life V should 



1 These words are used in the autobiography (1876): Life and 

 Letters, i. 86. See also the following passage in the letter written 

 to Hooker in July, 1844, the month in which Darwin finished the 



E2 



