Page - 129 - in The Origin of Species

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