NATURAL SELECTION 188 



tions. It is incredible that the descendants of two 

 organisms, which had originally differed in a marked 

 manner, should ever afterward converge so closely as to 

 lead to a near approach to identity throughout their whole 

 organization. If this had occurred, we should meet with 

 the same form, independently of genetic connection, re- 

 curring in widely separated geological formations; and 

 the balance of evidence is opposed to any such an 

 admission. 



Mr. Watson has also objected that the continued 

 action of natural selection, together with divergence of 

 character, would tend to make an indefinite number 

 of specific forms. As far as mere inorganic conditions 

 are concerned, it seems probable that a sufficient number 

 of species would soon become adapted to all considerable 

 diversities of heat, moisture, etc. ; but I fully admit that 

 the mutual relations of organic beings are more impor- 

 tant; and as the number of species in any country goes 

 on increasing, the organic conditions of life must become 

 more and more complex. Consequently there seems at 

 first sight no limit to the amount of profitable diversifi- 

 cation of structure, and therefore no limit to the number 

 of species which might be produced. We do not know 

 that even the most prolific area is fully stocked with 

 specific forms: at the Cape of Good Hope and in Aus- 

 tralia, which support such an astonishing number of 

 species, many European plants have become naturalized. 

 But geology shows us that from an early part of the 

 tertiary period the number of species of shells, and that 

 from the middle part of this same period the number of 

 mammals, has not greatly or at all increased. What then 

 checks an indefinite increase in the number of specie?? 



