| → that from an early part of the 1872 |
| at least within the whole immense 1861 |
| that from an early part of the long 1866 1869 |
|
| → that from the middle part of this same period the number 1866 1869 1872 |
| probably, 1861 |
|
|
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 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
divergence of
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
goes on increasing, the organic conditions of life
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 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 naturalised. But geology shows us,
→that from an early part of the
tertiary
the number of species of shells,
→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 species? The amount of life (I do not mean the number of specific forms) supported on
area must have a limit, depending so largely as it does on physical
therefore, if an area be inhabited by very many species, each or nearly each species will be represented by few individuals; and such species will be liable to extermination from accidental fluctuations in the nature of the seasons or in the number of their enemies. The process of extermination in
cases
be rapid, whereas the production of new
|
