240 CLASSICS OF MODERN SCIENCE 

 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 structure in the modified descend- 

 ants 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 sometimes 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, those 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 orig- 

 inally diflfered in a marked manner, should ever afterwards converge 

 so closely as to lead to a near approach to identity throughout their 

 whole organisation. If this had occurred, we should meet with 

 the same form, independently of genetic connection, 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 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, &c. ; 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 

 sight no limit to the amount of profitable diversification of structure, 

 and therefore no limit to the number of species which might be pro- 

 duced. 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 naturalised. But geology shows 



