ORIGIN OF SPECIES. 141 



way of inheritance. Yet it is only by the medium of her- 

 edity that Natural Selection can possibly act. 



5. Whilst it is undeniable that the individuals composing 

 any species vary more or less amongst themselves, there is 

 no proof that the variability of any species is indefinite. On 

 the contrary, there are reasons to believe that each species 

 is bounded by an uncertain but definite range of variability. 

 The extreme terms of this range may lie very far apart, but 

 between these runs somewhere a normal line or '* line of 

 safety," which is occupied by those individuals which may 

 be regarded as the type of the species. The doctrine how- 

 ever, of the evolution of species by natural selection de- 

 mands our assent to the belief that the variability of a species 

 is indefinite. 



6. The theory of the evolution of species by natural selec- 

 tion implies of necessity that one species can only be con- 

 verted into another through the medium of a great number of 

 successive forms, graduating into one another, each member 

 of the series differing from its immediate neighbours in 

 but minute characters. If, therefore, any existing species 

 has descended from any pre-existing species, there must at 

 one time have existed between the two species a graduated 

 series of intermediate forms. When we consider the enor- 

 mous number of living animals and plants, and the still 

 more enormous number of extinct forms which we know, or 

 may infer, to have existed in past time, it becomes clear — 

 if evolution be true — that the number of minutely inter- 

 mediate forms must have been incalculably great. We 

 have therefore the clear right to expect that Palaeontology 

 should reveal to us such intermediate forms, amongst the 

 vast series of fossil remains with which we are acquainted. 

 We cannot, however, in any case point to such forms. It 

 is quite true that there are many instances in which fossil 

 animals may be regarded as intermediate forms between 

 great groups of living forms, as missing links in the 

 zoological chain. Such intermediate forms, however, are 



