THE EVOLUTION OF UVING BEINGS. 135 



Selection.if notinterferedwithbycrossing.inevitably 

 ends with extinction because selection can act through 

 extermination only, and by thus diminishing the num- 

 ber of differently constituted types, decreases the possi- 

 bihty of crossing and thus the formation of new types, 

 which formation not only means the origin of new 

 species, but also the possibiUty of adaptation, as the 

 only way in which a Linneon can adapt itself to new 

 circumstances, is a putting into the world of new tj^pes. 



All this agrees perfectly with what we saw that hap- 

 pens, in the appearance and gradual extinction of new 

 classes of plants or animals, so that we are justified to 

 conclude: 



Crossing was the origin of the new classes ; selection, the 

 result of extermination by the struggle for life, the cause 

 of their gradual extinction. 



Such extinction of classes must proceed continuously, 

 until a happy meetingbetween two sufficiently different- 

 ly constituted gametes, causes the origin of a new class. 



We will therefore have periods of new-formations of 

 classes and periods of gradual extinction of such classes. 



In the present time we Uve in a period, in which the 

 extinction of many classes is almost completed, and in 

 which no new classes are formed. 



This is easily demonstrated for the vascular plants 

 by a glance at Scott's system: 



Equisetales. 



o I. J I Pseudobomiales. 



Sphenopsida < ^ , , „ , 



' Sphenophyllales. 



Psilotales. 



