XIX.] MORPHOLOGY. 239 



the animal scale as any of the Invertebrata. It belongs to 



the same class of facts that the stem and the arms of the Crinoids. 



Crinoids are jointed in a nearly similar way. 



The conclusions arrived at in this chapter may be thus 

 summed up : — 



All the facts of organic morphology are consistent with Summary 



of Til pi'^ 



the adaptation of structure to function ; but there are 

 many of the facts which are not cases of the law of 

 adaptation, and cannot be referred to it : such as (to 

 mention only a few examples) the resemblance, in a 

 greater degree than is reqmrccL Inj any imrfose of adaptation, 

 between the sexes of the human species, between homo- 

 logous parts like the hands and feet, and between the leaf- 

 bearing and the flower-bearing axes of the Umbelliferae. 



These are what Darwin calls correlations of form. It Correla- 

 ought not to surprise us to find this principle of correlation adaptation 

 pervading organic morphology. The wonder would be distinct, 

 rather if we did not find laws of the kind. But what I 

 wish to lay emphasis on is, that correlation in organic 

 morphology is totally distinct from adaptation to function, 

 and is much more nearly akin to the correlations of 

 crystalline morphology. IsTo doubt a correlation of form 

 may serve a purpose. For instance, the correlation, 

 amounting to identity of form, between the external 

 organs on both sides of the body among nearly all the 

 Vertebrata and Articulata is much more convenient than 

 any unsymmetrical arrangement could be. But this will 

 not apply to the correlation between the hands and the 

 feet. The fact that the toes are the same in number as 

 the fingers, is, as I have endeavoured to show, a case of 

 pure correlation, which has nothing to do with adaptation. 

 If this appears doubtful, a proof of it, which may be 

 almost called an experimental proof, is afforded by the 

 fact that the hands and the feet habitually vary together, 

 and sometimes present similar monstrosities. These facts 

 can, I think, be only compared to the laws of crystalline Crystalline 

 morphology, that edges and angles which are similarly ganic'mor. 

 related to similar axes are themselves similar ; and that ptology, 

 when the normal form of the crystalline species is modi- 



