THE GRAPTOLITES. 371 



When the Zones are worked out more fully than is the 

 case at present, we shall have a far better gauge of " Geo- 

 logical Time " than that founded upon the crude estimates 

 made by measuring thicknesses of strata. 



Lastly, the study of graptolites may possibly throw 

 some light upon climatic change. I have already en- 

 larged upon this elsewhere (24), and pointed out that the 

 separation of graptolitic deposits from non-graptolitic ones 

 amongst the Stockdale shales of the Lake District, the 

 deposits themselves being lithologically similar, is most 

 readily explicable by climatic change. The argument 

 would be stronger had microscopic examination and 

 chemical analyses of the strata been made, and I should 

 be glad to supply any one who cares to look into this 

 question, which is one of some interest, with material for 

 such examinations. 



In conclusion, the above notes will be sufficient to 

 show the importance which the graptolitoidea have 

 assumed not only to the geologist but also to the biologist. 

 That they differ in any remarkable respect, as regards 

 their teachings, from any other group of fossils is doubtful. 

 Their special utility lies in the fact that owing to their 

 characters they are preserved in sufficient numbers to 

 allow collectors to obtain a large suite of specimens of 

 almost every species with little difficulty ; the result is that 

 further advance has been made in their study than in that 

 of many other groups which like them are only preserved 

 in the fossil state. One word to the biologists. We are 

 often told that fossils are of little use on account of the 

 absence of soft parts, though biologists have not been 

 much hampered by this when dealing with the Vertebrata. 

 But to compensate for the want of soft parts, we are furnished 

 with a countless supply of specimens whose order of appear- 

 ance and disappearance we are able to a large extent to ascer- 

 tain, and this is what the biologist can never obtain by con- 

 fining his attention to recent organisms. From them he has 

 been able to ascertain that evolution occurs; how it occurs 

 is left for the palaeontologist to describe. That the study 

 of these organisms as pursued up to the present has not 



