303 GEOLOGICAL DISTKIBUTION. 



here, as elsewhere, it should be remembered that the break almost 

 everywhere existing between the Tertiary and Cretaceous series of 

 deposits is a profound one, and covers not impossibly a lapse of 

 time fully equal to that which is measured by either of the two pe- 

 riods here mentioned, or even greater. It is, therefore, in no way 

 very surprising that a family still in its prime at the beginning, or 

 even near the end, of the Cretaceous period should have become 

 practically extinct before the beginning of the Tertiary, in the in- 

 terval between which there may have been, as Darwin characteris- 

 tically observes, "much slow extermination." 



The doctrine advanced by many of the earlier geologists and 

 paleontologists, and still held by a few, that the duratioii of spe- 

 cies, or groups of species, is uniformly defined the world over by a 

 sharp and iniiexible line, is, in the light of our present knowledge 

 of facts, untenable, and will not stand the barest test of logical 

 examination. It cannot for a moment be conceived that the con- 

 junction of the physical and vital forces should have so acted as to 

 simultaneously convert favourable into unfavourable conditions of 

 existence for the entire surface of the earth ; as far as the widely- 

 distributed marine forms of life are concerned, nothing short of a 

 complete upheaval and laying dry of the sea-bottom could have 

 brought about such a condition, and, even granting such a consen- 

 taneous upheaval to have actually taken place, which probably no 

 geologist will for an instant admit, the overflow of the oceanic 

 waters would have afforded a safe harbour to many forms that might 

 have been displaced from their native habitat. Extermination or 

 extinction of the larger animal groups, especially if wide-spread, 

 will have been almost invariably preceded by displacement ; unfa- 

 vourable conditions for existence, whichever way they may have been 

 brought about, will tend to promote migration, and, consequently, 

 when a certain group of animals becomes extinct at a given period 

 of time, at any one locality or region, we may confidently look for 

 its survival somewhere else, possibly in some very distant region. 

 Instances of such survival, in favoured localities, after the nearly 

 complete extermination of the race, we see in the Californian ammo- 

 nite and Australian belemnite already referred to, in the genera Tri- 

 gonia and Pholadomya amon^ the acephalous mollusks, in the genus 

 Pentacrinus, and in Ceratodus, among fishes. The dinosaurian rep- 

 tiles, whose range in Europe only exceptionally extends to the top 



