HARDWICKE'S SCIENCE -GOSSIP. 



97 



THE MIGHTY DEEP. 



By A. RAMSAY. 



CCORDING to E. 



Forbes and others, it 

 is a convenient hy- 

 pothesis to assume that 

 each species has arisen 

 from a single centre, 

 so that each species, 

 each genus, and each 

 fauna occupies an area 

 which is complete in 

 itself. Thus, a species 

 may spread from a particular centre as far as it can find 

 suitable conditions. If such conditions were universal, 

 there is no reason why the species should not spread 

 into eveiy sea and exclude all other forms. There 

 are, however, always restrictions, so that each species 

 has a limited distribution, and if it spreads over most 

 of the space occupied by the ocean, it generally occurs 

 more or less sporadically, owing to alterations of 

 condition having driven it from certain parts. The 

 distribution of each species is, in most cases, a clue 

 to the distribution of the particular set of conditions 

 under which it lives. Hence species which find the 

 conditions for living on the sea bottom in deep water 

 will spread over that area of deep water so far as the 

 suitable conditions are sufficiently continuous. If 

 there is a break in the distribution of such conditions, 

 whether caused by the shallowing of the water or in 

 any other way, the further extension of the species 

 will be arrested. Most marine organisms have 

 powers of locomotion during some portion of their 

 lives, a circumstance which, combined with the trans- 

 porting power of currents, must not be overlooked. 

 The continuation of the species in time implies the 

 continuance of the conditions suitable to it, a circum- 

 stance which should always be present to the minds of 

 both paleontologists and geologists when endeavouring 

 to synchronize old sea bottoms. In all oceans of the 

 present period, and, as may be assumed, in all oceans 

 of past periods, there is and has been a similar con- 

 currence of physical conditions. The main difference 

 in time have been variations in the distribution of each 

 No. 161. 



set of conditions relatively to one another, while both 

 in time and in space these have been accompanied by 

 variations in the facilities afforded to each species in 

 passing from one area to another. This has caused 

 local groupings of species into faunas and floras, and 

 the preoccupation of the ground frequently prevents 

 a species from localizing itself where the physical 

 conditions are favourable for it. This preoccupation 

 is often a barrier to the spread of a species. Generally 

 the persistence of a fauna associated with a particular 

 set of conditions, whether in space or in time, indicates 

 the persistence of these conditions. For instance, 

 during the post-pliocene period, a particular assem- 

 blage of mollusks frequented the shallow waters of 

 the British isles. All these, or at any rate nearly all, 

 now occur on the east shore of N. America ; while 

 a much smaller proportion exist on the present shores 

 of Britain. Currents will not account for this altera- 

 tion in the distribution ; but the existence of a line 

 of shallow water on a coast extending from the 

 British isles to America would. The destruction of 

 this coast-line and modifications of the conditions on 

 the British shores with the continuance of such con- 

 ditions on the shores of America would explain the 

 remaining phenomena. This illustration (which is 

 only one of many that could be cited) shows that 

 the occurrence of the same fossils in the strata of two 

 distant areas indicates the occurrence of similar 

 conditions in both areas, but not that the faunas 

 are synchronous : the question of contemporaneity 

 should be based on other evidence. So, again, the 

 relative ages of strata cannot be deduced solely from 

 the similarity or dissimilarity of their fossils. It 

 is essential that the conditions connected with each be 

 taken into account, in order to determine this point. 

 If all the species now living were regarded as 

 fossils, and if local lists were made of them with a 

 view to comparing them with older faunas, some 

 would have a more ancient facies than others ; and a 

 geologist would, we suspect, assign them to different 

 periods. Facts indicate that geological provinces 

 existed in past periods analogous to those of the 



