II.] INCIPIENT STRUCTURES. 31 



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

 natural selection of these favourable variations has alone 

 lengthened the neck of the 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: 1 "It is demonstrable 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 



eight times the strain w 7 hich gravitation puts on its struc- 



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



