EFFECTS OF NATURAL SELECTION 129 



As all the modified descendants from a common and widely- 

 diffused species, belonging to a large genus, will tend to par- 

 take of the same advantages which made their parent success- 

 ful in life, they will generally go on multiplying in number as 

 well as diverging in character; this is represented in the dia- 

 gram by the several divergent branches proceeding from (A). 

 The modified offspring from the later and more highly im- 

 proved branches in the lines of descent, will, it is probable, 

 often take the place of, and so destroy, the earlier and less im- 

 proved branches : this is represented in the diagram by some 

 of the lower branches not reaching to the upper horizontal 

 lines. In some cases no doubt the process of modification 

 will be confined to a single line of descent, and the number 

 of modified descendants will not be increased; although the 

 amount of divergent modification may have been augmented. 

 This case would be represented in the diagram, if all the 

 lines proceeding from (A) were removed, excepting that 

 from a^ to a". In the same way the English race-horse and 

 English pointer have apparently both gone on slowly diverg- 

 ing 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, o", f and m", which, from 

 having diverged in character during the successive genera- 

 tions, will have come to differ largely, but perhaps unequally, 

 from each other and from their common parent. If we sup- 

 pose 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 ; 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 doubtful or at least into well-defined species. Thus the 

 diagram illustrates the steps by which the small differences 

 distinguishing varieties are increased into the larger differ- 

 ences distinguishing species. By continuing the same process 

 for a greater number of generations (as shown in the dia- 

 gram in a condensed and simplified manner), we get eight 

 species, marked by the letters between o" and »i", all de- 

 scended from (A). Thus, as I believe, species are multiplied 

 and genera are formed. 



E— HC XI 



