NATURAL SELECTION 



6. Most of the larger and some small groups extend 

 through several geological periods. 



7. In each period, however, there are peculiar groups, 

 found nowhere else, and extending through one or several 

 formations. 



8. Species of one genus, or genera of one family occurring 

 in the same geological time, are more closely allied than those 

 separated in time. 



9. As, generally, in geography no species or genus occurs 

 in two very distant localities without being also found in 

 intermediate places, so in geology the life of a species or 

 genus has not been interrupted. In other words, no group 

 or species has come into existence twice. 



10. The following law may be deduced from these facts : 

 Every species has come into existence coincident both in space and 

 time with a pre-existing closely allied species. 



This law agrees with, explains, and illustrates all the facts 

 connected with the following branches of the subject : 1st, 

 The system of natural affinities. 2d, The distribution of 

 animals and plants in space. 3d, The same in time, including 

 all the phenomena of representative groups, and those which 

 Professor Forbes supposed to manifest polarity. 4th, The 

 phenomena of rudimentary organs. We will briefly endeavour 

 to show its bearing upon each of these. 



The Form of a true system aj 



by this Law 



If the law above enunciated be true, it follows that the 

 natural series of affinities will also represent the order in 

 which the several species came into existence, each one having 

 had for its immediate antitype a closely allied species existing 

 at the time of its origin. It is evidently possible that two or 

 three distinct species may have had a common antitype, and 

 that each of these may again have become the antitypes from 

 which other closely allied species were created. The effect of 

 this would be, that so long as each species has had but one 

 new species formed on its model, the line of affinities will be 

 simple, and may be represented by placing the several species 

 in direct succession in a straight line. But if two or more 

 species have been independently formed on the plan of a 



