THE EVOLUTION OF LIVING BEINGS. 133 



and Angiosperms is generally known ; also their gradu- 

 al diminution in types (except in the latter group, 

 which is yet flourishing) until but a few remain, or total 

 extinction follows. 



If we compare the Cycadophytes of Mesozoic times, 

 with their highly developed Bennettitales, with the few 

 Cycadaceae of to day, or the Lycopodiales of the Coal- 

 measures, with their giant Lepidodendrons, with the 

 miserable Lycopods of to day, or the Equisetales of the 

 Coal-measures, with their numerous giant Calamites, 

 with the few horse-tails of the present time, or the fossil 

 Reptils and Ammonites of the Jura with the recent 

 ones, the exactness of the facts mentioned above, sub 2 

 and 3, is apparent. 



Now all this coincides splendidly with what we see 

 happening after a cross between two differentLinneons,viz. 

 istly. appearance, among the progeny of the hybrids, 



of a very large number of different forms. 

 2dly. gradual decrease of that large number of diffe- 

 rent forms until but a few remain within each 

 Linneon. 



Soon after the formation of a large number of diffe- 

 rent forms from a cross, these continue to intercross, 

 leading thus to a constant increase of new types, but 

 gradually this intercrossing ceases, either by isolation, 

 sterility or by any other obstacles among which aversion 

 may play a considerable role, and so smaller groups, Lin- 

 neons, find themselves together as communities to which 

 intercrossing, if such continues, is limited. If it does not 

 continue, mere segregation will rapidly cause an increa- 

 se of the homozygotes in proportion to the heterozy- 



