36 The Treasures of Siluria. [Jan., 



tinct statements of the cosmical (i.e. universal) theory of creations 

 and extinctions. For instance, " all beings are subject to external 

 conditions, favourable and unfixvourable, which assist in the produc- 

 tion of an average longevity." " Extinction of life is commonly 

 slow, continuous, individual, and sometimes is more rapid than 

 replacement from without, or than by acts of creation. Sudden 

 acts of extermination are exceptional, brief in time, and limited in 

 space." Again, the author divides the causes of extinction into two 

 classes, (1) mechanical and (2) physiological. The former includes 

 oscillation of level, climate, &c.; and the latter, such matters as 

 supjdy of food, overcrowding, epidemics, &c. 



AVith all this most students of the modern school will agree ; 

 but we imagine that our readers will be as surprised as ourselves to 

 find amidst so much of the *' uniformity " philosophy the following 

 sentence: — "The causes of extinction are in universal operation. 

 They are cosmical. Silurian life was discontinued everywhere at 

 the same time, proximately." Surely, if this statement includes the 

 actual fact, mechanical and physiological causes of extinction may 

 as well be neglected, for their operation must have been too re- 

 stricted to leave any impress on the geological record. 



As a corollary of his cosmical theory. Dr. Bigsby states that 

 " There is no example, as far as I know, of a Silurian community 

 rising, by migration or otherwise, into Devonian or Carboniferous 

 strata ; but single species do, and somewhat largely, just as we see 

 in every epoch up to the present." Our knowledge is not yet 

 sufficiently advanced to enable us to make many definite state- 

 ments on this subject ; but we may remind Dr. Bigsby that his 

 favourite (and deservedly great) authority, M. Barrande, has shown 

 that the two highest members of his Third Fauna (stages G and H) 

 present less^ strong connections with the Devonian system than do 

 the still older deposits E and F. Again, the author himself admits 

 that " whole communities have been known to return together to 

 the country they had long abandoned," and he quotes Mr. Godwin- 

 Austen's descrijition of " this kind of repossession in the Paleozoic 

 rocks near Boulogne." 



From these considerations the author passes naturally to the 

 question of recurrence, which he defines to be " the reappearance 

 of a plant or animal in a zone of rocks higher than that in which it 

 was first observed. It implies progress upwards, either on the 

 same spot or on another hy migration." Used in this sense, it is 

 remarkable how many species may be termed rcciu'rent, — what 

 a large number occur on more than one horizon. 



In illustration of the subject. Dr. Bigsby has constructed a 

 table showing a synoptical view of Sihirian life in this relation 

 as far as was known in 18G5. This table shows us that out of the 

 59(18 Silurian species (exclusive of the Primordial) whose places are 



