332 M. F. J. Pictet on the present state of the question as to 



of the fauna of Tereb. diphya of the Tyrol, and of that which 

 MM. de Verneuil and E. Favre have discovered in Andalusia. 

 Both these are of great importance, and may furnish some of the 

 elements that we require for the definitive solution. 



In the meanwhile I will conclude with a few theoretical reflec- 

 tions on the manner in which we should receive and discuss these 

 unexpected facts. 



Every one who seeks to enrich natural history with new facts 

 labours more or less under the dominion of certain general and 

 theoretical data, which cause him to see but one end to be attained 

 and one course to follow. Few, indeed, escape this influence, 

 which, it must be admitted, is most frequently a necessary excitant. 

 But it is necessary to guard against the exaggerations of this 

 principle, so as to avoid the danger of subordinating the results 

 of direct observation to preconceived ideas. Allow me to call 

 your attention now to some of these rocks which are to be 

 shunned. 



For my own part, I am quite agreed with those who believe 

 that the development and the succession of organisms have always 

 been and still are under the empire of perfectly regular laws. 

 The function of science is to endeavour to discover these; and 

 the further it advances the more brilliantly does this regularity 

 manifest itself. I by no means take part with those who believe 

 in irregularity and disorder, and whose tendency is to seek for 

 apparent exceptions, the inanity of which is usually soon shown 

 by the progress of observation. 



But whilst taking this regularity as our basis, we must recog- 

 nize the fact that the law T s are much more complex than had been 

 supposed. A few years ago we still believed in strongly marked 

 lines, in great catastrophes, and in the sudden succession of 

 faunas which had no material tie between them. We were 

 brought up in these notions ; and it has required all the teaching 

 of numerous facts to show us that the faunas must have become 

 modified more or less gradually, and with considerable slowness, 

 under the influence of varied circumstances, like those which 

 induce analogous effects in the existing seas. Now these cir- 

 cumstances are not identical at all times and in all places, which 

 renders the probability that things took place exactly in the 

 same manner in all geological basins infinitely small. The his- 

 tory of the succession of organisms must consequently be com- 

 plex and modified by local facts ; it is not probable that we shall 

 be able to formulate it in a very simple manner applicable to all 

 cases. 



In presence of the question which now engages us, I think it 



