46 PARALLELISM IN DEVELOPMENT. 



CHAPTER V. 



PARALLELISM IN DEVELOPMENT. 



In tlie course of tlie tliree preceding chapters, it has been 

 to a great extent our aim to sliow that certain animals 

 may resemble one another verj closely in general external 

 appearance, or may possess certain peculiar structural 

 features in common, without being in any way iutimately 

 related; thus rendering it evident that such similarities of 

 form or structure have been independently acquired, and 

 are not inherited from a common ancestor. It has been 

 shown, for instance, that mammals belonging to several 

 distinct orders, or to different families of the same order, 

 may assume such a marked external resemblance to the 

 common mole as to be designated iu popular language by 

 the same general title ; while in other cases a more or less 

 striking approximation to the type of the ordinary liedge- 

 hog has resulted from the independent development of 

 very similar spines in totally distinct groups. Furthermore, 

 it has been jiointed out in the first of the tliree chajaters 

 that large tusks of very similar form may be independently 

 developed in the jaws of totally different distinct groups 

 of mammals ; and even that certain extinct reptiles have 

 acquired tusks which are almost indistinguishable from 

 those of some of the carnivorous mammals, there being no 

 direct relationship between the members of these groups. 



With regard to the external similarities of form in the 

 above-mentioned instances, we have seen reason to believe 

 that their inducing cause has Iseen either similarity of 

 habit or the need of protection ; while in the case of the 

 tusks the similarity may in certain instances be due to 

 the necessity for efiicient offensive weapons. Be their 

 causes, however, what they may, it is evident that such 

 resemblances among animals, which are, so to speak, 

 accidental, indicate what may be termed a kind of parallel 



