300 STUDIES, SCIENTIFIC AND SOCIAL 



subject to a somewhat altered set of conditions, and by 

 further variation and selection may become the parent of 

 a number of subordinate species. It will now have 

 become a dominant genus, occupying an entire continent, 

 or perhaps even two or more continents, spreading on all 

 sides till it meets with competing forms better adapted to 

 the conditions which there prevail. 



Such a genus may continue to exist during long geolo- 

 gical epochs. Bats of the genus Vespertilio lived in the 

 Eocene period, and still range over all the globe, while 

 fossil land shells of the genus Pupa, hardly distinguishable 

 from some now living, are found in the ancient carboni- 

 ferous deposits. Generally, however, a time comes to 

 every genus when either physical changes, or competing 

 forms, or new enemies, are too much for it, and it begins 

 to lose its supremacy. First one, and then another of its 

 species dwindle away and become extinct, till at last two 

 or three only remain. Sometimes these soon follow the 

 others, and the whole genus dies out, as thousands of 

 genera have died out during the long course of the earth's 

 life-history ; but it also often happens that a few species 

 continue to maintain themselves in areas where they are 

 removed from the influences that have exterminated their 

 fellows. Thus the mudfish of Queensland (Ceraiodus 

 FoTsteri) and the Trigonia, a bivalve mollusc of the Aus- 

 tralian seas are the only living representatives of genera 

 which lived in the Triassic period. 



I have now, I think, shown that one of the most general 

 objections to natural selection as producing new species — 

 namely, that there are enormous chances against the right 

 kind of variations occurring just when they are required — 

 is utterly fallacious, by proving that there is ample varia- 

 tion of every kind constantly occurring among animals in 

 a state of nature. It has also been shown that many 

 different kinds of variation are occurring at the same 

 time, and in endlessly varied combinations, so that any 

 required combination of characters could be selected as 

 well as any single character. And when we consider the 

 extreme slowness of almost all the changes of conditions 



