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in character from their original stocks； without either having given off any fresh branches or races。
After ten thousand generations； species （A） is supposed to have produced three forms； a10； f10； and m10； which； from having diverged in character during the successive generations； will have come to differ largely； but perhaps unequally； from each other and from their common parent。 If we suppose the amount of change between each horizontal line in our diagram to be excessively small； these three forms may still be only well…marked varieties； or they may have arrived at the doubtful category of sub…species； but we have only to suppose the steps in the process of modification to be more numerous or greater in amount； to convert these three forms into well…defined species： thus the diagram illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences distinguishing species。 By continuing the same process for a greater number of generations （as shown in the diagram in a condensed and simplified manner）； we get eight species； marked by the letters between a14 and m14； all descended from （A）。 Thus； as I believe； species are multiplied and genera are formed。
In a large genus it is probable that more than one species would vary。 In the diagram I have assumed that a second species （I） has produced； by analogous steps； after ten thousand generations； either two well…marked varieties （w10 and z10） or two species； according to the amount of change supposed to be represented between the horizontal lines。 After fourteen thousand generations； six new species； marked by the letters n14 to z14； are supposed to have been produced。 In each genus； the species； which are already extremely different in character； will generally tend to produce the greatest number of modified descendants； for these will have the best chance of filling new and widely different places in the polity of nature： hence in the diagram I have chosen the extreme species （A）； and the nearly extreme species （I）； as those which have largely varied； and have given rise to new varieties and species。 The other nine species （marked by capital letters） of our original genus； may for a long period continue transmitting unaltered descendants； and this is shown in the diagram by the dotted lines not prolonged far upwards from want of space。
But during the process of modification； represented in the diagram； another of our principles； namely that of extinction； will have played an important part。 As in each fully stocked country natural selection necessarily acts by the selected form having some advantage in the struggle for life over other forms； there will be a constant tendency in the improved descendants of any one species to supplant and exterminate in each stage of descent their predecessors and their original parent。 For it should be remembered that the competition will generally be most severe between those forms which are most nearly related to each other in habits； constitution； and structure。 Hence all the intermediate forms between the earlier and later states； that is between the less and more improved state of a species； as well as the original parent…species itself； will generally tend to become extinct。 So it probably will be with many whole collateral lines of descent； which will be conquered by later and improved lines of descent。 If； however； the modified offspring of a species get into some distinct country； or become quickly adapted to some quite new station； in which child and parent do not come into competition； both may continue to exist。
If then our diagram be assumed to represent a considerable amount of modification； species （A） and all the earlier varieties will have become extinct； having been replaced by eight new species （a14 to m14）； and （I） will have been replaced by six （n14 to z14） new species。
But we may go further than this。 The original species of our genus were supposed to resemble each other in unequal degrees； as is so generally the case in nature； species （A） being more nearly related to B； C； and D； than to the other species； and species （I） more to G； H； K； L； than to the others。 These two species （A） and （I）； were also supposed to be very common and widely diffused species； so that they must originally have had some advantage over most of the other species of the genus。 Their modified descendants； fourteen in number at the fourteen…thousandth generation； will probably have inherited some of the same advantages： they have also been modified and improved in a diversified manner at each stage of descent； so as to have become adapted to many related places in the natural economy of their country。 It seems； therefore； to me extremely probable that they will have taken the places of； and thus exterminated； not only their parents （A） and （I）； but likewise some of the original species which were most nearly related to their parents。 Hence very few of the original species will have transmitted offspring to the fourteen…thousandth generation。 We may suppose that only one （F）； of the two species which were least closely related to the other nine original species； has transmitted descendants to this late stage of descent。
The new species in our diagram descended from the original eleven species； will now be fifteen in number。 Owing to the divergent tendency of natural selection； the extreme amount of difference in character between species a14 and z14 will be much greater than that between the most different of the original eleven species。 The new species； moreover； will be allied to each other in a widely different manner。 Of the eight descendants from （A） the three marked a14； q14； p14； will be nearly related from having recently branched off from a14； b14 and f14； from having diverged at an earlier period from a5； will be in some degree distinct from the three first…named species； and lastly； o14； e14； and m14； will be nearly related one to the other； but from having diverged at the first commencement of the process of modification； will be widely different from the other five species； and may constitute a sub…genus or even a distinct genus。 The six descendants from （I） will form two sub…genera or even genera。 But as the original species （I） differed largely from （A）； standing nearly at the extreme points of the original genus； the six descendants from （I） will； owing to inheritance； 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 consideration）； 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 descended 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 have descended from some one species of an earlier genus。 In our diagram； this is indicated by the broken lines； benea