136 GEOLOGICAL DISTRIBUTION. 



earth's geological history, the Cambrian. But it must be borne in 

 mind that in the Cambrian formation we have only what is seem- 

 ingly the oldest fossiliferous formation, and that the ancestral forms 

 of both ocean-lilies and star-fishes lie buried in rock deposits of 

 undeterminably older age. If, on the hypothesis of evolution, we 

 uphold the inter-derivation, or derivation from one another, of these 

 two forms, then it is but fair to assume that the crinoid, which is 

 structurally the lowest, appeared at a period considerably anterior 

 to the star-fish, which must have required for its specialisation a 

 no inconsiderable lapse of time. And it is a singular fact, and one 

 strikingly confirmatory of this view of relationship, that we have in 

 both these forms certain peculiarities of structure which effect a 

 sort of transition from the one to the other. Thus, in some of the 

 fixed crinoids the plume or tuft separates from the column after 

 a certain period of existence, and then leads an independent exist- 

 ence, to all appearance a stellarid (the Comatula). Conversely, the 

 ofl[icers of the late " Travailleur " deep-sea dredging expedition 

 obtained off the coast of Spain, and from depths respectively of 

 nineteen hundred and sixty and twenty-six hundred and fifty 

 metres, two individuals of a new genus of star-fish (since named 

 by PeiTier Caulaster peduncularis), which exhibited on the dorsal 

 surface a true peduncle, demonstrated to be absolutely homologous 

 with the stalk of the crinoid. Yet, despite this obvious relation- 

 ship, it is not a little surprising that no pedunculated star-fish has 

 thus far been found fossil, nor any comatulid crinoid to antedate 

 the Jurassic period (Antedon). 



The Cambrian Mollusca comprise representatives of five of the 

 six classes that now inhabit the seas, namely, the Brachiopoda, 

 Acephala, Pteropoda, Gasteropoda, and Cephalopoda. Here again, 

 therefore, we have an apparent simultaneous appearance of lower 

 and higher forms; but, as before, we must look to a much earlier 

 period for the ancestral traces, if any have been preserved, of the 

 first or most primitive type. The genetic relationships of these 

 various molluscan groups cannot, in the present state of the science, 

 be determined with any degree of certainty ; but, if a low degree 

 of organisation indicates antiquity, which certainly appears to be 

 the case with many groups of animals, then it may be fairly assumed 

 that tlie Brachiopoda were the first to appear. It is a surprising 

 fact in the history of these animals, and one which is, perhaps, not 



