610 A. A. W. HUBEECHT. 



over by natural selection, may lead to a diminution of the 

 lateral stems, and to an increase of the dorso-raedian one. 



This, in my opinion, as will be more fully developed below, 

 was the case in the ancestors of the Chordata, traces of this 

 struggle and of the competing structural elements being duly 

 preserved. 



If we suppose the bilateral symmetry to be established in 

 one of the lower representatives of the Metazoa, and the type 

 to go on increasing in length in the course of generations, 

 then this increase indeed exposes it to very different and per- 

 haps more numerous dangers and enemies than would 

 threaten it were the same bulk concentrated in a spherical or 

 radial circumference. And if, even in the latter case, injuries 

 to the individual might prove fatal were it not provided with 

 strong powers of regeneration (cf. Star-fishes, Ophiurids, 

 Crinoids, &c.), still it needs no comment that, when bilateral 

 symmetry and increase in length so considerably enlarges the 

 surface which is open to attacks, and so enormously facilitates 

 the rupture of the individual, or the severing of parts by rapa- 

 cious enemies preying upon it, similar regenerative powers 

 are none the less required to insure the persistence of the 

 type. 



These dangers, continually threatening the exist- 

 ence of the individuals, and thus injurious to the 

 species, counteracted as they are by regenerative pro- 

 cesses (power of reproduction of lost parts), I hold to be 

 at the base of all those cases of metamery in the 

 animal kingdom which do not fall under the head of 

 strobilation, the latter being comparatively rare with respect 

 to the former. Incipient metamery, once established by this 

 cause, may further differentiate in the most diverse directions 

 (heteronomous segmentation, &c.), even after the absolute 

 cessation of the causes that in the first instance have 

 provoked it. 



The explanation has, moreover, the advantage of being 

 applicable to radial as well as to serial metamery. 



These propositions must now be more fully developed. The 



