70 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



divisions must necessarily, if we accept the ordinary teachings of 

 Evolution, have become materially differentiated, so that each 

 modern natural division has many of its generic types peculiar. 



The present distribution of all terrestrial organisms, as has been 

 pointed out already by Huxley, Wallace, and other writers on the 

 subject, is the combined result of several different factors. Of these 

 the original centre of dispersion in the case of each organic unity, 

 such as a family, genus, or species, is one ; the distribution of land 

 and water, firstly, at the time of dispersion, secondly, since that time 

 are two others ; whilst powers of migration and ability to live under 

 varied conditions are of great importance, and it is notorious that 

 the last two factors are as diverse in different organisms as I shall 

 endeavour to show that the first is. 



As regards origin, there is an important point in which mammals 

 and birds, most reptiles and batrachians, probably all insects and 

 arachnida, and all land-plants differ from freshwater fishes and 

 Crustacea, and from both freshwater and land-mollusca. The forms 

 in the first category are in all probability derived from terrestrial 

 or freshwater ancestors, differing very widely from them in structure, 

 so widely, indeed, that the ancestral types would have been classed in 

 distinct orders, or even classes. Even when there are marine 

 representatives, such as Cetacea, sea-snakes (^Hydropliidce)^ marine 

 turtles, and a few marine angiospermous plants, these are probably 

 descended from terrestrial or freshwater forms. On the other hand, 

 the. fishes, Crustacea, and mollusca found in rivers, and all land- 

 shells are in all likelihood derived from various marine stocks, and 

 some of them have living marine representatives belonging to the 

 same family. Thus freshwater Percidce, Scdmonidce, Clupeidce^ &c., 

 differ sometimes generically, often merely specifically from the forms 

 found in the sea, and when, as in the carps and in most existing 

 ganoids, whole families are confined to fresh water at the present day, 

 there can be no reasonable doubt that marine ancestors not differing 

 greatly in structure flourished at a former period. The derivation 

 of land-mollusca is similar, and will be dealt with presently. In the 

 case of terrestrial and fluviatile animals derived from marine 

 forms it is manifest that the geological date of origin of the 

 different genera of one family, or even of species of one genus, and 

 above all of different families was not necessarily the same, that is 

 their origin as land or freshwater animals may have taken place, 

 and in all probability did take jjlace, not merely at different periods 

 of the Earth's history, but in different parts of the land-area. 



