It was now clear that this model could have explained the supposed anomalies in 
the geographical distribution of animals and plants, such as the presence of 
very peculiar structures in isolated localities and islands. Such species were 
to be regarded either as very recent ones, which had had not time to spread, or 
as very old ones, which were survivors of an ancestral population. However, if 
the extinction of a species was established as a natural phenomenon, the mode of 
operation of which was observable in every natural station, another fundamental 
problem still remained unsolved : ‘is it possible that new species can be called 
into being from time to time, and yet that so astonishing a phenomenon can 
escape the observation of naturalists ?’ (67). Indeed, no one had ever observed 
such an astonishing event, and it was clear that Lyell's method of taking into 
account the agents at present in operation, in order to infer by analogy their 
action in the past, was not applicable to the problem of the appearance of new 
species. Lyell felt the need to explain why it was not possible either to 
observe the production of new species, or propound a mechanism, real or 
theoretical, for such an event. He embarked therefore on a very sophisticated 
order of considerations, closely resembling his friend Charles Babbage's 
favourite kind of calculations, and concluded that it would ‘require more than 
eight thousand years’ before a new species could be produced, or an old one 
extirpated, in a region of the extent of Europe, and within the class of mammals 
alone. Thus, no satisfactory answer could be given to the question of the 
production of new species ; Lyell was trapped in his balance of nature model, 
which had no power to allow newcomers to settle, and indeed took great care in 
eliminating them, either as invaders or as hybrids. Is, therefore, the 
introduction of new species a natural process, carried on by natural causes ?
 

we have some data to guide the conjecture, and to enable us to speculate with 
advantage, but it would be premature to anticipate such discussions, until we 
have laid before the reader an ample body of materials amassed by the industry 
of modern geologists (68).
 

No such discussions followed ; Lyell did return to this ambiguous conclusion in 
the third volume of the Principles, where he more explicitly declared that the 
appearance of new species is part of the ordinary course of nature, and again in 
some letters he later wrote to J. F. W. Herschel and to W. Whewell (69). Lyell's 
discussions of the species question had dealt comprehensively with Lamarckism, 
but had also reinforced the legitimacy of asking the still crucial question : if 
the introduction of a new species is part of the scheme of nature, how does it 
actually happen ? With the different solutions given to this question, we enter 
fully into the debates on species which animated the following decades of the 
century. As far as Lyell's contribution to the general discussion of the problem 
of species, and the wider dimension of biological debates in the 1820s and early 
1830s 

(67) C. Lyell, ibid., 179.

(68) C. Lyell, ibid., 183-4.

(69) C. Lyell, Principles, iii, 30 ; and loc. cit. (16), i, 464-9 ; ii, 2-5.