VIII.] HOMOLOGIES. 185 



The other kind of serial repetitions, namely, those of 

 the vertebral column, are explained by Mr. Spencer as the 

 results of alternate strains and compressions acting on a 

 primitively homogeneous cylinder. The serial homology of 

 the fore and hind limbs is explained by the same writer as 

 the result of a similarity in the influences and conditions 

 to which they are exposed. Serial homologues so formed 

 might be called, as Mr. Ray Lankester has proposed, 

 " homoplastic." But there are, it is here contended, abun- 

 dant reasons for thinking that the predominant agent in 

 the production of the homologies of the limbs is an internal 

 force or tendency. And if such a power can be shown to 

 be necessary in this instance, it may also be legitimately 

 used to explain such serial homologies as those of the cen- 

 tipede's segments and of the joints of the backbone. At 

 the same time it is not, of course, pretended that external 

 conditions do not contribute their own effects in addition. 

 The presence of this internal power will be rendered more 

 probable if valid arguments can be brought forward against 

 the explanations which Mr. Herbert Spencer has offered. 



Latercd liomolorjy (or bilateral symmetry) is the resem- 

 blance between the right and left sides of an animal, or of 

 part of an animal; as, e.g., between our right hand and our 

 left. It exists more or less at one or other time of life in 

 all animals, except some very lowly organized creatures. 

 In the highest animals this symmetry is laid down at the 

 very dawn of life, the first trace of the future creature 

 being a longitudinal streak — the embryonic " primitiA'e 

 groove." Tliis kind of liomology is explained by Mr. 

 Spencer as the result of the similar way in which condi- 

 tions affect the right and left sides respectively. 



Vertical liomolofjy (or vertical symmetry) is the resem- 



