m ANT SINGLE FORMA TION. 333 



pelago would migrate, and no closely consecutive record of 

 their modifications could be preserved in any one forma- 

 tion. 



Very many of the marine inhabitants of the archipelago 

 novf range thousands of miles beyond its confines; and 

 analogy plainly leads to the belief that it would be chiefly 

 these far-ranging species, though only some of them, 

 which would oftenest produce new varieties; and the vari- 

 eties would at first be local or confined to one place, but if 

 possessed of any decided advantage, or when further modi- 

 fied and improved, they would slowly spread and supplant 

 their parent-forms. When such varieties returned to their 

 ancient homes, as they would differ from their former state 

 in a nearly uniform, though perhaps extremely slight degree, 

 and as they would be found imbedded in slightly differ- 

 ent sub-stages of the same formation, they would, accord- 

 ing to the principles followed by many palseontologists, be 

 ranked as new and distinct species. 



If then there be some degree of truth in these remarks, 

 we have no right to expect to find, in our geological for- 

 mations, an infinite number of those fine transitional forms 

 which, on our theory, have connected all the past and 

 present species of the same group into one long and branch- 

 ing chain of life. We ought only to look for a few links, 

 and such assuredly we do find — some more distantly, some 

 more closely, related to each other; and these links, let 

 them be ever so close, if found in different stages of the 

 same formation, would, by many paleeontologists, be 

 ranked as distinct species. But I do not pretend that I 

 should ev3r have suspected how poor was the record in the 

 best preserved 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. 



OS THE SUDDEN" APPEARANCE OE WHOLE GEOUPS OE 

 ALLIED SPECIES. 



The abrupt manner in which whole groups of species 

 suddenly appear in certain formations, has been urged by 

 several paleeontologists — for instance, by Agassiz, Pictet, 

 and Sedgwick — as a fatal objection to the belief in the 



