74 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



the air, whilst other forms, such as Ariophanta, have rather large soft 

 membranous eggs, quite unsuited for wind-transport to any distance, 

 Exce^Dt with the minute forms, I suspect that transport by wind 

 across the sea to any distance is extremely rare, and it is in 

 favour of this view that the species found on oceanic islands are 

 almost always peculiar, testifying to a long period of isolation. 

 On the other hand, how rapid may be the migration of some fresh- 

 water forms was shown by Dreissensia iiolymorpha, which, in about 

 a century, spread over a large part of Europe from the Caspian to 

 Scotland and the south of Erance. 



If it be the fact, as undoubtedly it is, that different subdivisions 

 of the animal and vegetable kingdoms have originated at different 

 geological periods, the next important question is whether, inde- 

 pendently of evidence from fossil remains, there is any clue to 

 difference of age, whether any characters exist by means of which 

 groups of more or less ancient origin can be recognized. It is 

 probable that, as a general rule, the most recent groups are those in 

 which the fewest breaks occur, and in which the distinctions 

 between families and genera are most difficult to define ; for these 

 distinctions become better marked as, in course of time, links die out 

 through exposure to the varying effects of change in climate and 

 the distribution of land, the development of enemies and the 

 struggle for existence. It is quite true that much depends upon the 

 power of each group of organisms to resist the influence of change ; 

 thus omnivorous animals would have greater facilities for obtaining 

 subsistence, should their usual food no longer be procurable, than 

 forms that feed invariably on fruit or vegetables, or flesh or 

 insects, and animals with the power of flight, as already mentioned, 

 may escape by migration, or those adapted for an aquatic life by 

 swimming, whilst creatures unable to fly or swim are overwhelmed 

 by floods or destroyed by change of climate, famine, or enemies. 

 Still, after taking all these circumstances into account, and bearing 

 in mind that the process of evolution appears much slower in some 

 groups of organisms than in others, we shall probably not be far 

 wrong in concluding that, as a general rule, groups of living beings 

 with all the members nearly related are of more recent origin than 

 those in which there are broad distinctions between the different 

 genera and families. 



Amongst the whole of the Ycrtebrata there is, I believe, no large 

 group all the members of which are so closely connected together as 

 the passerine * birds. They comprise more than 6000 known species, 



* Not including picarians such as Tici, Coccyges, Cypseli, &c. 



