THE ORIGIN OF SPECIES. 47 



elimination and segregation of nearly allied forms — 

 such as varieties, sub-species, and closely-related or rep- 

 resentative species — also in a general way for their geo- 

 graphical association and present range, is compara- 

 tively easy, is apparently within the bounds of possi 

 bility. Could we stop here we should be fairly con- 

 tented. Butj to complete the system, to carry out the 

 principles to their ultimate conclusion, and to explain 

 by them many facts in geographical distribution which 

 would still remain anomalous, Mr. Darwin is equally 1 

 bound to account for the formation of genera, families, 

 orders, and even classes, by natural selection. He 

 does " not doubt that the theory of descent with 

 modification embraces all the members of the same 

 class," and he concedes that analogy would press the 

 conclusion still further ; while he admits that " the 

 more distinct the forms are, the more the arguments 

 fall away in force." To command assent we natu- 

 rally require decreasing probability to be overbalanced 

 by an increased weight of evidence. An opponent 

 might plausibly, ana perhaps quite fairly, urge that 

 the links in the chain of argument are weakest just 

 where the greatest stress falls upon them. 



To which Mr. Darwin's answer is, that the best 

 parts of the testimony have been lost. He is confi- 

 dent that intermediate forms must have existed; that 

 in the olden times when the genera, the families, and 

 the orders, diverged from their parent stocks, grada- 

 tions existed as fine as those which now connect close- 

 ly related species with varieties. But they have passed 

 and left no sign. The geological record, even if all 

 displayed to view, is a book from which not only many 



