44 



THE VOYAGE OF H.M.S. CHALLENGER. 



marginalis ; if checked in its second, as an Orhitolites duplex ; and if checked in its 

 third, as the earlier (fossil) form of Orhitolites comjjlanata. 



Second, that all these ancestral types are still living ; and that, so far as we know 

 the external conditions of their existence, they are precisely the same as those of the 

 ■completed form. 



Third, that the absence of any distinguishable differentiation in the parts of the 

 sarcodic body of even the most " complex " Orbitolines, seems to make their physiological 

 relation to their " environment " precisely the same as that still held by the whole series 

 of ancestral forms. 



In considering: the oenetic relations of these several forms, and the ciixumstances 

 under which one has given origin to another, it is requisite to keep the distinction clearly 

 fl.nd constantly before the mind, between growth .md derolopmcnt ; — the former consisting 



Fig. 6. — Diagframmatio representation of 

 the sarcodic body ol Miliola. 



rt, Primordial segment. 



1-6, Successive segments, marked oil" l)y constric- 

 tions at intervals. 



^^z:Js- 



Fig. 7.— Shell of Pniemplix, sliowing its successive scptii 

 traversed by pores at regular inter\als. 



in the extension of the original fabri-- ini the same plan, by the multiplication of similar 

 parts ; whilst the latter involves a change of plan, of which dissimilarity (consisting in 

 the supersession of the original homogeneousncss by heterogeneousness) is the essential 

 feature. Thus, a Cornuspira that begins its life in the form of a conical tube coiled into 

 a nautiloid spire, raaj' expand itself, as it grows, into a flattened tube, I'apidly increasing 

 iu the breadth of its mouth, without any greater change than we see in an Amceha, 

 which, at one time an almost spherical lump of protoplasm, quickly flattens itself out 

 into a disk with pseudopodial extensions. But if, instead of flattening itself out, the 

 body of a Cornuspira should undergo constriction at intervals, as in fig. 6, and should 

 form at each constriction a partial septum across its tube, we recognise a new departure 

 that constitutes an advance in devehipment ; whilst as long as further additions are 

 made upon this new (Spiroloculine) plan, the process is one of growth only. AVhen, 



