296 NATURAL HISTORY. 



probable that any variation would simultaneously appear 

 in many individuals of a species ; and we have at any rate 

 no evidence to show that this ever occurs. 



(3) The theory of the origin of species by means 

 of natural selection, in the third place, implies that the 

 production of any given species from any pre-existing 

 species can only be effected by gradual modification, 

 and therefore through the intervention of a long series 

 of intermediate or transitional forms. Moreover, the 

 transitional forms by which we should pass from a given 

 species to the pre-existing species from which it was 

 developed, must, on the theory of natural selection, be 

 so closely related to one another as to render it difficult 

 to distinguish them. In other words, if we had before 

 us all the forms by which one species had been gradually 

 converted into another, we should not have the slightest 

 difficulty in recognising the distinctness of the individuals 

 forming the extreme terms of the series; but the 

 individuals standing between the extremes would pass 

 into one another by such fine gradations as to render 

 their separation almost or quite impossible. It seems 

 also clear that, in the modification of any one species 

 into any other, the total number of the individuals of 

 intermediate or transitional form must greatly exceed the 

 total number of individuals contained in the original 

 species and the new species put together. Now, if all 

 species of animals, living and extinct, have been produced 

 by gradual modification from pre-existing species, we 

 ought to find abundant evidence of the existence of the 

 infinite number of transitional forms postulated by the 

 theory of natural selection. In fact, as these transitional 



