NATURAL SELECTION 103 



other so closely that they would have all to be classed under the 

 same genus; and thus the descendants of two distinct genera would 

 converge into one. But it would in most cases be extremely rash to 

 attribute to convergence a close and general similarity of struc- 

 ture in the modified descendants of widely distinct forms. The 

 shape of a crystal is determined solely by the molecular forces, 

 and it is not surprising that dissimilar substances should some- 

 times assume the same form; but with organic beings we should 

 bear in mind that the form of each depends on an infinitude of 

 complex relations, namely on the 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 degree 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 their forms determined through equally complex 

 relations. 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 con- 

 nection, recurring 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 natu- 

 ral selection, together with divergence of character, would tend to 

 make an indefinite number of specific forms. As far as mere inor- 

 ganic 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 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 



