2 THE VOYAGE OF H.M.S. CHALLENGER 



the diatomaceous growth from the area of deposition while these intrusive layers were 

 being formed. 



That the comparison of recent and fossil or semi-fossil species is very useful as a means 

 of elucidating structural peculiarities cannot be doubted, yet the determination of species 

 is a matter of great difficulty. Thus although two given forms may be readily seen to 

 belong to the same genus, the points of difference between them may be merely varietal 

 and not specific, hence it becomes necessary to give careful consideration to the real 

 significance of these points. To those who hold that all organic forms are but modifica- 

 tions or evolutions of a monad or primitive cell, the existence of species is enigmatical and 

 such differential features are of but secondary moment ; while, on the other hand, to those 

 who regard the slightest deviation from a typical form as sufficient justification for the 

 creation of species, these characters become of great value. In any case, however, although 

 it is often impossible to decide whether two closely allied organisms differ in a varietal or 

 specific manner, the omission to record the existence of distinct structures, which tend to 

 make the history of the given type more complete, is unjustifiable. For this reason the 

 iudication of every notable form will frequently be made, but modifications will very often 

 be treated as varietal, and new species will never be founded save on the presence of 

 two or more interesting differences existing side by side. Where these are established, or 

 new varieties recorded, the arguments that lead to such a course will always be given. 



With regard to the geographical and bathymetrical distribution of Diatoms, no 

 definite results can yet be laid down on account of the very limited condition of our 

 knowledge of marine forms. As to the former, however, the desirability of registering the 

 locality from which every organism has been obtained as a means of facilitating future 

 work is apparent. The bathymetrical range is a question of even greater difficulty, as 

 only living species, and the depth at which they are found, can be taken into account. It 

 may here be noted that the advantages arising from a careful record of the depth from 

 which Crustacea, corals, madrepores, &c, are obtained, are very great, as frustules recog- 

 nised to be living by the presence of the endochrome are often found adhering to their 

 surfaces, and thus facts may often be gathered that go to aid in the determination of the 

 limits in depth of vegetable life. 



The importance of prosecuting investigations in this direction is so great that a means 



of conducting experiments was suggested by me in March 1871. ' I had repeatedly 



observed that, after exposing a vessel of water covered with a glass plate to the light 



for some weeks, small Diatoms and other unicellular Algae appeared in it, and the 



same result was obtained whether the water were fresh or marine. As this growth could 



only have been due to germs pre-existing in the water, since carefully prepared artificial 



sea-water led to entirely negative conclusions, it is very probable that if, with a suitable 



apparatus, sea-water could be drawn from various depths, for example, at intervals of 



1 Esame microscopico c note critiche su un campione di fango Atlantico ottenuto nella spedizione del 

 "Porcupine" nell' anno 1869, Atti. Accad. Pontif. d. nuov. Lincei, vol. xxiv. pp. 16-29, 1871. 



