ORGANIC DEPENDENCE AND DISEASE 25 



There is also a stabilization that affects an entire fauna 

 when the members of the assemblage are all in balance with 

 their external and internal control; and so a single fauna 

 may endure for a long period without change of complex- 

 ion. Thus the invertebrate shelled fauna of the Mississip- 

 pian or Lower Carboniferous marine limestones which 

 spread over Colorado, New Mexico and northern Texas, 

 shows such uniformity of character through a very long 

 lapse of time ; and in the Middle Devonian Hamilton period, 

 when shallower water prevailed, there is a similar continu- 

 ity of organic character without variation, throughout 

 nearly a thousand feet of shale which must represent a 

 period of many thousand years. The fauna must, however, 

 eventually succumb, that is, yield by evolutionary or intrin- 

 sic variation, or by extrinsic change ; shallowing or deepen- 

 ing of the sea, change of climate, a hundred outside influ- 

 ences, to its surroundings; just as many of the long-lived 

 species must yield, or at any rate have yielded, to a resist- 

 ance too great even for their conservatism to overcome. It 

 is again to be emphasized that it is protected, encased and 

 immobile life alone that achieves such long endurance ; the 

 conservative and sheltered types. The mobile and locomo- 

 tory animals have at no time in the earth 's history evinced 

 long life without change. 



These conclusions are so well established that we may 

 rightly look to them for light upon the interpretation of 

 certain tendencies to rest and unrest, conservatism and im- 

 pulsive change, in human society, and while it may not seem 

 very appropriate to speculate on the further bearing of 

 this theme, it must be said in looking back over the field of 

 organic history, that the value of the product must be in 

 terms of the worth of the type conserved or broken; that 

 is, worth in the sense of highest attainment in functional 

 grade and in the approach to mentality. In such a sense a 

 lobster is better than an oyster because it is of a vastly 

 more refined grade of structure ; and though the oyster has 



