THE EVOLUTION OF LIVING BEINGS. 99 



themselves the result of a cross would never have 

 originated, so that Linneons owe their origin to the occa- 

 sional possibility of a cross and their persistence to the 

 bars which nature, as a rule, keeps closed to prevent inter- 

 crossing, and but occasionally opens. 



The view, that distinctness of the Linneons is caused 

 by bars to intercrossing with other Linneons, is greatly 

 supported by the fact, that in cases as those of the Wil- 

 lows where the Linneons are, as we all know, very badly 

 limited, this bar against intercrosssing with other 

 Linneons does not only not exist, but such intercros- 

 sing is, on the contrary, favored (see Chapter XI). 



Whenever nature allows crossing, evolution sets in ; when- 

 ever crossing is made impossible it stops as soon as the 

 segregation initiated by the cross, is at an end. 



As the bars, separating the Jordanons within the Lin- 

 neons, are usually open, evolution within the Linneon, 

 the formation of so called new varieties" is a pheno- 

 menon occuring daily, and it is stopped permanently 

 only by the cropping up of apogamy. 



As on the contrary, the bars between the different 

 Linneons are usually closed, the origin of anewLinnean 

 species is a phenomenon of much rarer occurrence and 

 very frequently stopped definitely by the innate sterility 

 of two Linneons. 



The production of new Linneons, usually called spe- 

 cies", is generally designated as progressive evolution, 

 the production of new Jordanons, usually called va- 

 rieties" is frequently called degressive evolution. 



