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 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, 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 backbone.

At the same time it is not, of course, pretended that external conditions do not contribute their own effects in addition.