II.] INCIPIENT STRUCTURES. 31 



length of the neck. If, as Mr. Danvin contends, the 

 natural selection of these favourable variations has alone 

 lengthened the neck of tlie giraffe by preserving long- 

 necked individuals during droughts ; similar variations, 

 in other similarly-feeding forms, ought similarly to have 

 been preserved, and so have lengthened the neck of such 

 other Ungulates by similarly preserving them during the 

 same droughts. 



(2.) It may be also objected, that the power of reaching 

 upwards, acquired by the lengthening of the neck and legs, 

 must have necessitated a considerable increase in the entire 

 size and mass of the body (larger bones requiring stronger 

 and more voluminous muscles and tendons, and these again 

 necessitating larger nerves, more capacious blood-vessels, 

 &c.), and it is very problematical whether the disadvantages 

 thence arising would not, in times of scarcity, more than 

 counterbalance the advantages. For a considerable increase 

 in the supply of food would be requisite on account of this 

 increase in size and mass, while at the same time there would 

 be a certain decrease in strength ; since, as Mr. Herbert 

 Spencer says:^ "It is (^monstrable that the excess of 

 absorbed over expended nutriment must, other things equal, 

 become less as the size of an animal becomes greater. In 

 similarly-shaped bodies, the masses vary as the cubes of the 

 dimensions; whereas the strengths vary as the squares of the 

 dimensions." . . . . " Supposing a creature which a year ago 

 was one foot high, has now become two feet high, while it 

 is unchanged in proportions and structure — what are the 

 necessary concomitant changes that have taken place in it? 

 It is eight times as heavy ; that is to say, it has to resist 

 eiglit times the strain which gravitation puts on its struc- 

 1 "Principles of Biology," vol. i. p. 122. 



