276 THE WORLD OE LIFE 



require a different shape in the articulating joints of the hips 

 and some change in the muscles; and this would be the more 

 important as the hind- and fore-legs now have unequal angular 

 motions when galloping, involving changed co-ordination in 

 all the connected parts, any failure in which would diminish 

 speed and thus be fatal to the varying individuals. Even 

 the blood-vessels and nerves of these various parts would re- 

 quire modifioations exactly adapted to the change in the other 

 parts ; and he urges that any individuals in Avhich all these 

 necessary variations did not take place simultaneously, would 

 be at a disadvantage and would not survive. To do his argu- 

 ment justice, I will quote one of his most forcible paragraphs. 



" The immense change in the ratio of fore-quarters to hind- 

 quarters would make requisite a corresponding change of ratio in 

 the appliances carr3dng on the nutrition of the two. The entire 

 vascular system, arterial and venous, would have to undergo succes- 

 sive unbuildings and rebuildings to make its channels everywhere 

 adequate to the local requirements, since any want of adjustment 

 in the blood-supply to this or that set of muscles would entail in- 

 capacity, failure of speed, and loss of life. Moreover, the nerves 

 supplying the various sets of muscles would have to be appropriately 

 changed, as well as the central nervous tracts from which they 

 issued. Can we suppose that all these appropriate changes, too, 

 would be, step by step, simultaneously made by fortunate spon- 

 taneous variations occurring along with all the other fortunate 

 spontaneous variations? Considering how immense must be the 

 number of these required changes, added to the changes above 

 enumerated, the chances against any adequate readjustments for- 

 tuitously arising must be infinity to one." 



Xow, this seems very forcible, and has, no doubt, con- 

 vinced many readers. Yet the argument is entirely fallacious, 

 because it is founded on the tacit assumption that the number 

 of the varying individuals is very small, and that the amount 

 of coincident variation is also both small and rare. It is 

 further founded on the assumption that the time allowed for 

 the production of any sufficient change to be of use is also 



