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



« one are often represented by closely allied families, genera 

 iind species peculiar to the other. jg 



f(^ The distribution of the organic world in time is very similar 

 .■><'to its present distribution in space. 



C. 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 

 ^^iijSame geological time are more closely allied than those sepa- 

 i^Iifrated in time. 



5^' As generally in geography no species or genus occurs in two 

 •fo i very distant localities without being also found in interme- 

 iffc,^iate places, so in geology the life of a species or genus has 

 {)<jrnot been interrupted. In other words, no group or species 

 ojjj'.has come into existence twice. 



y^. The following law may be deduced from these facts : — Every 

 g'l .species has come into existence coincident both in space and 

 J^\:,^t^me with a pre-existing closely allied species A6reiii.oi{tm J^oiob 



'"'This law agrees with, explains and illustrates' air tfi'e'mcis 

 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 in time, including all the 

 phsenomena of representative groups, and those which Professor 

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

 rudimentary organs. We will briefly endeavour to show 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 compound, and can only be represented by a 

 forked or many-branched line. Now, all attempts at a Natural 

 classification and arrangement of organic beings show, that both 



