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



which the circumnavigating voyage of the Challenger is the most extensive, that one- 

 half of the surface of the earth forms, at a certain depth below the surface of the sea, a 

 continuous region under very uniform conditions, which are not unfavourable to the 

 existence, nor to the moderate development of animal life. This region is peopled by a 

 fauna, not certainly of extreme poverty, and very special in its nature ; its specialty 

 consisting mainly in its great uniformity and in the prevalence of certain types. There 

 is every reason to believe that the existing physical conditions of this area date from a 

 very remote period, and that the present fauna of the deep-sea may be regarded as being 

 directly descended from faunse which have successively occupied the same deep-sea. In 

 the meantime, changes involving lesser depths have been accompanied by the appearance 

 and disappearance of the land and shallow-water faunse of the Jurassic, the Cretaceous, and 

 the Tertiary periods. That the present abyssal fauna is the result of progressive change 

 there can be no room for doubt ; but it would seem th,at in this case the progress has been 

 extremely slow, and that it has been brought about almost in the absence of those causes, 

 — such as minor and local oscillations of the crust of the earth producing barriei-s, and affect- 

 ing climate, — on which we are most inclined to depend for the modification of faunae. 



The discovery of the abyssal fauna, accordingly, seems ' to have given us an oppor- 

 tunity of studying a fauna of extreme antiquity, which has arrived at its present 

 condition by a slow process of evolution from which all causes of rapid change have been 

 eliminated. A careful study of such an assemblage of forms must in time do much to 

 throw light upon many difficult problems of distribution. Even now, with the vaguest 

 outline only before us, derived from a single line of scattered soundings, I am prepared 

 to admit a strong personal impression ujaon two points. 



I believe that the study of the abyssal fauna, revealing many delicate cliains of struc- 

 tural affinity linking the fauna of the present with that of the past, brings into 

 prominence a new mass of facts, morphological, ontological, and palseontological, in 

 powerful support of the doctrine of Evolution. On the other hand, it seems to me that 

 in this, as in all cases in which it has been possible to bring the question, however 

 remotely, to the test of observation, the character of the abyssal fauna refuses to give 

 the least support to the theory which refers the evolution of species to extreme variation 

 guided only by natural selection. Species are just as distinctly marked in the abyssal 

 fauna as elsewhere, each species varying within its definite range as each species appears 

 to have varied at all times, past and present. If all the species living on the floor of the 

 ocean were, and had always been, in a state of instability, acted upon by external in- 

 fluences, and perpetually passing by insensible gradations into other species, it seems 

 certain that the general impression drawn from a fauna such as that of the abyssal region 

 must have been one of indefiniteness and transition. This is not the case. Transition 

 forms, linking species so closely as to cause a doubt as to their limit, are rarely met with. 

 There is usually no difficulty in telling what a thing is. 



