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 structure 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
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, 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
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