THE FACTORS OF ORGANIC EVOLUTION. 76^ 



assume that they vary togetlicr, but we are "warranted in asserting 

 that they can have no tendency to vary together. And what are the 

 implications in cases where increase of a structure can be of no sei^vice 

 unless there is concomitant increase in many distant structures, which 

 have to join it in performing the action for which it is useful ? 



As far back as 186-4 {Principles of Biology, § 166) I named in 

 illustration an animal carrying heavy horns — the extinct Irish elk ; 

 and indicated the many changes in bones, muscles, blood-vessels, 

 nerves, composing the fore-part of the body, which would be required 

 to make an increment of size in such horns advantageous. Here let 

 me take another instance — that of the giraffe : an instance which I 

 take partly because, in the sixth edition of the Origin of Species, 

 issued in 1872, Mr. Darwin has referred to this animal when effectu- 

 ally disposing of certain arguments urged against his hypothesis. lie 

 there says : 



" In order that an animal should acquire some structure specially and largely 

 developed, it is almost indispensable that several other parts should be modified 

 and co-adapted. Although every part of the body varies slightly, it does not 

 follow that the necessary parts should always vary in the right direction and to 

 the right degree " (p. 179). 



And in the summary of the chapter, he remarks concerning the ad- 

 justments in the same quadruped, that " the pi-olonged use of all the 

 parts together with inheritance will have aided in an important man- 

 ner in their co-ordination" (p. 199) : a remark probably having refer- 

 ence chiefly to the increased massiveness of the lower part of the neck; 

 the increased size and strength of the thorax required to bear the ad- 

 ditional burden ; and the increased strength of the fore-legs required 

 to carry the greater weight of both. But now I think that further 

 consideration suggests the belief that the entailed modifications are 

 much more numerous and remote than at first appears ; and that the 

 greater part of these are such as cannot be ascribed in any degree to 

 the selection of favorable variations, but must be ascribed exclusively 

 to the inherited effects of changed functions. Whoever has seen a 

 giraffe gallop will long remember the sight as a ludicrous one. The 

 reason for the strangeness of the motions is obvious. Though the 

 fore-limbs and the hind-limbs differ so much in length, yet in gallop- 

 ing they have to keep pace — must take equal strides. The result is 

 that at each stride, the angle which the hind-limbs describe round 

 their centre of motion is much larger than the angle described by the 

 fore-limbs. And beyond this, as an aid in equalizing the strides, the 

 hind part of the back is at each stride bent very much downward and 

 forward. Hence the hind-quarters appear to be doing nearly all the 

 work. Now a moment's observation shows that the bones and muscles 

 composing the hind-quarters of the giraffe, perform actions differing 

 in one or other way and degree, from the actions performed by the 

 homologous bones and muscles in a mammal of ordinary proportions, 



