124 THE VOYAGE OF H.M.S. CHALLENGER. 



stems in what was originally a uniform plexus are preserved ; whereas, in ancestral 

 Nemertea, two lateral longitudinal trunks in the plexus were undoubtedly characteristic 

 features. 



That one medio-dorsal stem in this plexus, in which all the impressions made by out- 

 ward agencies on both halves of the body might be concentrated, and from whence the 

 corresjDonding movements might be regulated, will more fully answer the purpose than 

 two lateral stems, however they may be united by an intervening plexus, is a priori 

 probable, and would explain the first impulse towards the formation of such a longitu- 

 dinal concentration in the uniform plexus. 



And when once such a dorso-inedian stem is present, in addition to two lateral ones, 

 a struggle for supremacy, presided over by natural selection, may lead to a diminution 

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



This, in my opinion, as will be more fully developed below, was the case in the ances- 

 tors of the Chordata, traces of this struggle and of the competing structural elements 

 being duly j)reserved. 



If we suppose the bilateral symmetry to be established in one of the lower re- 

 presentatives 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 perhaps 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 

 specimen 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 specimen, or the severing of 

 parts by rapacious enemies preying upon it, similar regenerative powers are none the 

 less required to insure the persistence of the type. 



These dangers, continually threatening the existence of the specimens, and thus in- 

 jurious to the species, counteracted as they are by regenerative processes (power of repro- 

 duction of lost -peLvts), I hold to he at the base of all those cases of metamery in the animal 

 kingdom ivhich do not fall under the head of strohilation, the latter being comparatively 

 rare with respect to the former. Incipient metamery, once established by this cause, 

 may further dilFerentiate in the most diverse directions (heterouomous 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 radia] as well as' 

 to serial metamery. 



These propositions must now be more fully developed. The power of reproduction of 

 lost parts comes, without doubt, under the general laws of formation and growth. We 

 find it even in the lowest Protozoa. If the material which heredity has accumulated, either 

 in such a unicellular being or in the egg of a Metazoon, and out of which the elements of 



