DISPERSAL OF GRAPTOLITES 281 



to have been active creatures, and may have made their 

 passage from place to place by their own muscular 

 exertions. That many of them certainly lived in the 

 locality in which they are now found is proved by the 

 occurrence of vast numbers of uninjured shells of the 

 same species, and in all stages of growth, crowded 

 together within very limited areas. 



Whatever explanation we adopt, there can be no 

 possible doubt that the transformation of Ammonites 

 from species to species took place with an altogether / 

 exceptional degree of rapidity, and it is to this unusual 

 plasticity, as well as to some exceptionally rapid means 

 of distribution, that they owe their value for geological 

 purposes. 



In the case of the G-raptolites, Professor Lap worth 

 has proposed to account for their wide distribution by 

 means similar to those conceived by Professor Walther 

 for the Ammonites. They usually occur in blackish 

 sediments supposed to owe their colour to the carbona- 

 ceous matter of decayed seaweeds, and it is suggested 

 that these were floating forms like those of the modern 

 Sargasso Sea. If, as Professor Lapworth supposes, the 

 Graptolites grew attached to these seaweeds, their wide 

 distribution might be brought about by winds and 

 currents. Some species are supposed, on very good 

 evidence, to have shared with some of their existing 

 relatives among the Hydrozoa the possession of a 

 pneumatophore, or swimming bladder, like that of the 

 " Portuguese man-of-war "; beneath this the gonothecae, 

 or reproductive individuals of the organism, appear to 

 have been situated. Such forms might float for long 

 long distances (Figs. 86 and 87). 



No doubt the explanations of Professors Walther and 

 Lapworth offer a partial solution of the difficulty, but the 

 evidence, particularly in the case of Ammonites, is so 



