CHAPTER IX 



THE ORIGIN AND EXTINCTION OF SPECIES 



One of the most important problems involved, in the complicated 

 mechanism of evolution is that of the rise and progress of species. 

 Passmg over the question of spontaneous generation and the origin 

 of organic life, we have to ask, How are species formed ? We con- 

 sidered this question briefly in the first chapter, where we noted 

 that different species are evolved from a common ancestor ; that 

 transformations of type are effected in the first place by variation ; 

 in the second place by selection, which causes the best variation 

 under given conditions to survive ; in the third place by heredity, 

 which consoUdates the result obtained by selection by fixing and 

 maintaining it. Variation, selection, and heredity are the three 

 great factors iu the evolution of species, the three great laws of 

 organic life. 



Researches which have been made in regard to the evolution 

 of the land-snails of the Celebes, notably the studies of Sarasin, 

 afford a good example of what we may term spatial, as distinct 

 from temporal, evolution. These researches show that, since the 

 later Tertiary period, a remarkable evolution of these land-snails 

 has taken place. A number of new species have been evolved, 

 and their evolution has proceeded pari passu with the retreat of 

 the sea, and with the consequent growth of the island. The 

 present species form, in their totality, a chain in which the dif- 

 ferent species are aU connected by intermediary links, and the 

 result is that if we consider them in their order of progression 

 from the first to the last, we find before us, not the well-differen- 



129 9 



