CHAP. IV.] NATURAL SELECTION 1 ". 89 



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

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

 all become, excepting (F), extinct, and have left no descendant.-;. 

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

 descendants 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 

 diagram, 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 14 , 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 U ) will not V>e 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 generally 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. 



I see no reason to limit the process of modification, as now 

 explained, to the formation of genera alone. If, in the diagram, 

 we suppose the amount of change represented by each successive 

 group of diverging dotted lines to be great, the forms marked a 14 

 to /> u , those marked 6 14 and /", and those marked o 14 to m l \ will 

 form three very distinct genera. We shall also have two very 



