14 THE FACTORS OF ORGANIC EVOLUTION. 



comitant variation in co-operative parts that are near 

 together if we do not find it in parts which, though 

 belonging to different tissues, are so closely united as teeth 

 and jaws if we do not find it even when the co-operative 

 parts are not only closely united, but are formed out of the 

 same tissue, like the crab s eye and its peduncle; what shall 

 we say of co-operative parts which, besides being composed 

 of different tissues, are remote from one another? Not only 

 are we forbidden to assume that they vary together, 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 service 

 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 1864 (Principles of Biology, 166) I named 

 in illustration an animal carrying heavy horns t!ie 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 effectually disposing of certain arguments 

 urged against his hypothesis. He there says : 



&quot; 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 &quot; (p. 179). 



And in the summary of the chapter, he remarks concerning 

 the adjustments in the same quadruped, that &quot;the pro 

 longed use of all the parts together with inheritance will 

 have aided in an important manner in their co-ordination &quot; 

 (p. 199) : a remark probably having reference chiefly to 



