XXVI.] THE KATE OF VARIATION. 345 



of years ; previous to which time it was condensing out of 

 its original nebula. Of course this calculation does not 

 make the slightest pretension to precision, but all that is 

 needed for the present argument is the order of the mag- 

 nitude.^ Five hundred mUlions of years is a period so 

 overpowering to the imagination, that it is no wonder if 

 most readers think it equivalent for all practical purposes 

 to eternity. I think, however, that for the present purpose 

 this is very doubtful. 



Darwin justly mentions the greyhound as being equal The gi-ey- 

 to any natural species in the perfect co-ordination of its ^^"a^- ^^ 

 parts, " all adapted for extreme fleetness and for running species ; 

 down weak prey." ^ Yet it is really an artificial species,^ 

 formed by long-continu.ed selection under domestication ; 

 and there is no reason to suppose that any of the varia- 

 tions which have been selected to form it have been 

 other than gradual and almost imperceptible. Suppose 

 that it has taken five hundred years to form the greyhound produced 

 out of his wild wolf-like ancestor : this is a mere guess, s^o'yearr 

 but it gives the order of the magnitude. Now, we have t>y slow 

 seen that the past duration of the earth has been about 

 five hundred millions of years, or only one million times 

 greater than this ; yet during that period all those changes 

 have taken place by which the highest animal — let us say 

 the elepliant — has been developed out of a Protozoon ; 

 or, if we admit the hypothesis of a separate ancestor for 

 the vertebrate and for every other fundamentally distinct 

 form, during that period the elephant has been developed 

 out of the earliest and most lowly organized fish. Now, if 

 it takes five hundred years to obtain the present race of 

 greyhounds from a woK-like ancestor by an accumulation 

 of slow variations, how long would it take to obtain an ^°^^,l°°,^ 



' ^ would the 



elephant from a Protozoon, or even from a tadpole-like production 



1 See the article ou Darwin's theory, in the North British Review for 

 June 1867. 



2 Darwin on Variation under Domestication, vol. ii. p. 221. 



3 This expression will be objected to. But, as I have alreadj' stated, I 

 think Darwin, in his Origin of Species, has proved that the distinction 

 between species and variety is only one of degree. 



