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



in their very highest grade of development. And yet, whilst these three types of form 

 are so well marked, and so constantly reproduced genetically, that the whole Challenger 

 collection (with an exception to be presently referred to) can be ranged under one or 

 another of them, yet even in the assemblage that is characterised by the most 

 complex type of structure, certain individuals are found, which, in the earlier stages 

 of their development, are no less characteristically representative of the lowest and 

 intermediate. But the fourth of these specific types, Orhitolites tenuissima, in which the 

 pedigree just now traced-out presents itself most completely and unmistakably, is not 

 only (so far as is yet known) remarkably constant in its characters, but, whilst con- 

 structed on the very simplest plan, is separated from Orhitolites marginalis (which is 

 precisely on the same grade of development with itself) by very sharply-defined 

 peculiarities of its own. And it is not a little remarkable that its habitat should be 

 almost entirely different from that of the other three ; its home being apparently in the 

 cold depths of the North Atlantic, whence it has strayed into the littoral zone of the 

 Iberian peninsula, and thence along the Mediterranean into the -^gean, where it 

 encounters a similar " outlier " of Orhitolites 7)%arginalis, which has probably found its 

 way thither through the Red Sea. 



III. " The only natural classification of the vast aggregate of diversified forms which 

 this group contains will be one which ranges them according to their direction and 

 degree of divergence from a small number of principal family types ; and any subordinate 

 groupings of genera and species which may be adopted for the convenience of description 

 and nomenclature, must be regarded merely as assemblages of forms characterised by the 

 nature and degree of the modifications of the original type, which they may have 

 respectively acquired in the course of genetic descent from a common ancestry." 



Of this principle, the evidence I have now to present of the genetic derivation of 

 the most complex and highly-specialised Orbitoline type from the simplest and 

 most generalised Milioline, will be found — to say the least — peculiarly illustrative ; 

 its special value as a " Study in the Theory of Descent " consisting in this, that whilst 

 the ancestral relations of the higher types of organisation are for the most part 

 evinced in transitory phases of development, of which few or even no ti'aces may remain 

 in the adult, we here find the whole genetic history distinctly recognisable in the 

 completed type. 



Having thus set forth what I regard as the principles on which alone a Natural 

 System of the Foraminifeka generally can be framed, I shall proceed to apply these in 

 the description I have now to give of the genus Orhitolites, and of the specific types 

 which my enlarged study of it now enables me to recognise. 



