Chap. X.] IN ANY SINGLE FORMATION. 77 



geological sections, had not the absence of innumerable 

 transitional links between the species which lived at 

 the commencement and close of each formation, pressed 

 so hardly on my theory. 



On the sudden Ajjpearance of whole Groups of 

 allied Species. 



The abrupt manner in which whole groups of species 

 suddenly appear in certain formations, has been urged 

 by several palaeontologists — for instance, by Agassiz, 

 Pictet, and Sedgwick — as a fatal objection to the belief 

 in the transmutation of species. If numerous species, 

 belonging to the same genera or families, have really 

 started into life at once, the fact would be fatal to the 

 theory of evolution tlirough natural selection. For the 

 development by this means of a group of forms, all of 

 which are descended from some one progenitor, must 

 have been an extremely slow process ; and the progeni- 

 tors must have lived long before their modified descen- 

 dants. But we continually overrate the perfection of 

 the geological record, and falsely infer, because certain 

 genera or families have not been found beneath a 

 certain stage, that they did not exist before that stage. 

 In all cases positive paleeontological evidence may be 

 implicitly trusted ; negative evidence is worthless, as 

 experience has so often shown. We continually forget 

 how large the world is, compared with the area over 

 which our geological formations have been carefully 

 examined ; we forget that groups of species may else- 

 where have long existed, and have slowly multiplied, 

 before they invaded the ancient archipelagoes of Europe 

 and the United States. We do not make due allowance 



