186 Mr. A. R. Wallace on the Law ivhich has regulated 



one are often represented by closely allied families, genera 

 and species peculiar to the other. 



Geology. 



5. The distribution of the organic world in time is very similar 



to its present disti'ibution in space. ' ' * 



6. Most of the larger and some small groups extend through 



several geological periods. 



7. In each period, however, there are peculiar groups, found no- 

 where 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 sepa- 

 rated in time. 



9. As generally iu geography no species or genus occurs in two 



very distant localities without being also found in interme- 

 diate 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. 2nd. The distribution of animals 

 and plants in space. 3rd. The same iu time, including all the 

 phenomena of representative groups, and those which Professor 

 Forbes supposed to manifest polarity. 4th. The phsenomena of 

 rudimentary organs. We will briefly endeavour to sihow its 

 bearing upon each of these. 



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 repre- 

 sented by placing the several species in direct succession in a 

 straight line. But if two or more species have been indepen- 

 dently formed on the plan of a common antitype, then the series 

 of affinities will be com]jound, and can only be represented by a 

 forked or many-branched line. Now, all attcm})ts at a Natural 

 classification and arrangement of organic beings show, that both 



