On The Origin of Species by Charles Darwin Chapter 10 Page 61

all the past and present species of the same group into one long and branching 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 palaeontologists, be ranked as distinct species.

But I do not pretend that I should ever 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.

5. On The Sudden Appearance Of Whole Groups Of Allied Species

The abrupt manner