LIFE ON THE EARTH. 201 



are lineal descendants of some other and generally 

 extinct species, in the same way as the acknowledged 

 varieties of any one species are the descendants of 

 that species. Furthermore, I am convinced that 

 natural selection has been the main but not the 

 exclusive means of modification. 7 



If asked how far he extends the doctrine of the 

 mutability of species, he replies : 



'The question is difficult to answer, because the 

 more distinct the forms are which we may consider, 

 by so much the arguments fall away in force. But 

 some arguments of the greatest weight extend very 

 far. All the members of whole classes can be con- 

 nected together by chains of affinities, and all can 

 be classified on the same principle, in groups sub- 

 ordinate to groups. Fossil remains sometimes tend 

 to fill up very wide intervals between existing orders. 

 Organs in a rudimentary condition plainly show that 

 an early progenitor had the organs in a fully deve- 

 loped state; and this in some instances necessarily 

 implies an enormous amount of modification in the 

 descendants. Throughout whole classes various 

 structures are formed on the same pattern, and at 

 an embryonic age the species closely resemble each 

 other. Therefore I cannot doubt that the theory 

 of descent with modification embraces all the mem- 

 bers of the same class. I believe that animals have 



