LIFE OF FLOWER 101 



brought about by the modification of structures origin- 

 ally more or less dissimilar towards a common type. 

 In other words, the same goal has been reached by two 

 different routes. 



An excellent example of this is offered by the de- 

 velopment of " cannon-bones " in the lower portion of 

 the limbs of the members of the horse tribe on the one 

 hand and those of the deer and antelopes on the other ; 

 the object of this lengthening and strengthening of this 

 part of the limb being in both instances the attainment of 

 increased speed. Whereas, however in the one instance 

 the cannon-bone is formed from one original element, 

 in the other it is the result of the fusion of two such 

 elements. In this case, indeed, the difference in the 

 structure of this part of the skeleton in the two groups 

 is so apparent as to leave no reasonable doubt as to the 

 remoteness of the affinity between their respective 

 ancestors. There is, however, a certain group of ex- 

 tinct South American hoofed mammals in which the 

 cannon-bone corresponds exactly in origin and structure 

 with that of the horse, from which it might be assumed 

 that the two animals were closely related, whereas, from 

 other evidence, we know that they are widely sundered. 

 Approximately similar structures are therefore in many 

 instances far from being indications of genetic affinity 

 between the animals in which they respectively occur. 

 Before the occurrence of this parallelism was recognised 

 by naturalists as an important factor in their develop- 

 ment, such resemblances were, however, frequently 

 regarded as indications of a common parentage, so that 

 animals which had comparatively little to do with one 

 another were brigaded as members of the same assemblage. 



