Chap. IV. Natural Selection. 95 



nearly at the extreme end of the original genus, the six descendants 

 from (I) will, owing to inheritance alone, differ considerably from 

 the eight descendants from (A) ; the two groups, moreover, are 

 supposed to have gone on diverging in different directions. The 

 intermediate species, also (and this is a very important considera- 

 tion), which connected the original species (A) and (I), have all 

 become, excepting (F), extinct, and have left no descendants. 

 Hence the six new species descended from (I), and the eight de- 

 scended from (A), will have to be ranked as very distinct genera, 

 or even as distinct sub-families. 



Thus it is, as I believe, that two or more genera are produced 

 by descent with modification, from two or more species of the same 

 genus. And the two or more parent-species are supposed to be 

 ■descended from some one species of an earlier genus. In our dia- 

 gram, this is indicated by the broken lines, beneath the capital 

 letters, converging in sub-branches downwards towards a single 

 point ; this point represents a species, the supposed progenitor of our 

 several new sub-genera and genera. 



It is worth while to reflect for a moment on the character of the 

 new species f u , which is supposed not to have diverged much in 

 character, but to have retained the form of (F), either unaltered or 

 altered only in a slight degree. In this case, its affinities to the 

 other fourteen new species will be of a curious and circuitous nature. 

 Being descended from a form which stood between the parent-species 

 (A) and (I), now supposed to be extinct and unknown, it will be 

 in some degree intermediate in character between the two groups 

 descended from these two species. But as these two groups have 

 gone on diverging in character from the type of their parents, the 

 new species (f 14 ) will not be directly intermediate between them, 

 but rather between types of the two groups ; and every naturalist 

 will be able to call such cases before his mind. 



In the diagram, each horizontal line has hitherto been supposed 

 to represent a thousand generations, but each may represent a 

 million or more generations ; it may also represent a section of the 

 successive strata of the earth's crust including extinct remains. We 

 shall, when we come to our chapter on Geology, have to refer again 

 to this subject, and I think we shall then see that the diagram 

 throws light on the affinities of extinct beings, which, though gene- 

 rally belonging to the same orders, families, or genera, with those 

 now living, yet are often, in some degree, intermediate in character 

 between existing groups ; and we can understand this fact, for the 

 extinct species lived at various remote epochs when the branching 

 lines of descent had diverged less. 



