178 THE GENESIS OF SPECIES. [CHAP. 



degree of aggregation in one longitudinal series, through 

 survival of the fittest aggregations. This may be so. It is 

 certainly an ingenious speculation, but facts have not yet 

 been brought forward which demonstrate it. Had they 

 been so, this kind of serial homology might be termed 

 " homogenetic." 



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 similaritj 7 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, 

 abundant 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 centipede's segments and of the joints of the back- 

 bone. 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. 



Lateral homology (or bilateral symmetry) is the re- 

 semblance 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 



