00 VBRQENVB OF OB ABA CTER. \-) 1 



variations which have arisen, these being due to causes far too 

 intricate to be followed out— on the nature of the variations 

 which have been preserved or selected, and this depends on 

 the surrounding physical conditions, and in a still higher de- 

 gree on the surrounding organisms with which each being has 

 come into competition — and lastly, on inheritance (in itself a 

 fluctuating element) from innumerable progenitors, all of 

 which have had theirforms determined through equally com- 

 plex relations. It is incredible that the descendants of two 

 organisms, which had originally differed in a marked man- 

 ner, 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 important; 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 no limit to the amount of 

 profitable diversification 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 Australia, 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 ter- 

 tiary period the nuuiber of species of shells, and that from 

 the middle part of this same pei-iod, the number of mam- 

 mals has not greatly or at all increased. What then checks 

 an indefinite increase in the number of species? The 

 amount of life (I do not mean the number of specific 

 forms) supported on an area must have a limit, depending 

 so largely as it does on physical conditions; therefore, if 

 an area be inhabited by very many species, each or nearly 



