284 ON THE ORIGIN OF SPECIES 



existence, as long as it lasts, is continuous. I a,ia 

 aware that there are some apparent exceptions to this 

 rule, but the exceptions are surprisingly few, so few 

 that E. Forbes, Pictet, and Woodward (though all 

 strongly opposed to such views as I maintain) admit its 

 truth ; and the rule strictly accords with my theory. 

 For as all the species of the same group have descended 

 from some one species, it is clear that as long as any 

 species of the group have appeared in the long suc- 

 cession of ages, so long must its members have con- 

 tinuously existed, in order to have generated either new 

 and modified or the same old and unmodified forms. 

 Species of the genus Lingula, for instance, must have 

 continuously existed by an unbroken succession of 

 generations, from the lowest Silurian stratum to the 

 present day. 



We have seen in the last chapter that the species of 

 a group sometimes falsely appear to have come in 

 abruptly ; and I have attempted to give an explanation 

 of this fact, which if true would have been fatal to my 

 views. But such cases are certainly exceptional ; the 

 general rule being a gradual increase in number, till the 

 group reaches its maximum, and then, sooner or later, 

 it gradually decreases. If the number of the species of 

 a genus, or the number of the genera of a family, be 

 represented by a vertical line of varying thickness, 

 crossing the successive geological formations in which 

 the species are found, the line will sometimes falsely ap- 

 pear to begin at its lower end, not in a sharp point, but 

 abruptly ; it then gradually thickens upwards, some- 

 times keeping for a space of equal thickness, and 

 ultimately thins out in the upper beds, marking the 

 decrease and final extinction of the species. This 

 gradual increase in number of the species of a group is 

 strictly conformable with my theory ; as the species of 

 the same genus, and the genera of the same family, can 

 increase only slowly and progressively ; for the process 

 of modification and the production of a number of 

 allied forms must be slow and gradual, — one species 

 giving rise first to two or three varieties, these being 



