1878.] HEMIPTERA OF ST. HELENA. 453 



period, there ought to be some genera represented (amongst those 

 which are not absolutely peculiar) which are of comparatively recent 

 origin ; but the facts are that out of 15 genera not peculiar, 10 are 

 actually Miocene 1 , and 4 others are of such wide distribution that, 

 though they have not yet been detected in Miocene formations, they 

 must be of Miocene age. The remaining genus may have either 

 been a later importation, or, what is as likely, originated en route. 

 The 25 peculiar genera are all, as I have stated, allied to Miocene, 

 or probably Miocene, families ; and their great number is an addi- 

 tional argument in favour of the long isolation and consequent early 

 settlement of St. Helena. 



The Hemiptera (so far as our defective knowledge of their geolo- 

 gical and geographical distribution goes) tell the same tale. Of the 

 13 genera not peculiar, 4 have at least actual, and 3 probable, 

 Miocene affinities ; of the remaining 6 we know less. Of the 8 

 peculiar genera, 4 have actual and 3 probable Miocene affinities. 



Again, we learn the same thing from the Arachuida. The 13 

 genera are all probably Miocene, 1 being and 5 belonging to families 

 known as Miocene. 



The 4 genera of terrestrial Mollusca are all Eocene ; and one has a 

 peculiar subgenus. These quite support the evidence learnt from 

 the insects. 



2. The aboriginal fauna did not arrive all at once, but the coloni- 



zation was spread over a lengthened period. 

 This is a question into which I need not enter at length, but 

 merely refer to Mr. Darwin's remarks on the inhabitants of oceanic 

 islands 2 , and point out how the great number of peculiar species in 

 St. Helena bears out his arguments. If all the inhabitants had 

 arrived simultaneously, their mutual relationships would have been 

 undisturbed, and the liability to modification consequently less- 

 ened by the absence of new forms of competition. Spread over a 

 lengthened period, the colonization of a new land must inevitably 

 upset the preexisting relationships of the colonists, and result in, as 

 we see in St. Helena, many modifications of form. 



3. The road by which the colonists travelled was not on a continuous 



land-surface. 



This, too, is a question not requiring lengthy consideration here. 



The absence of Batrachians and terrestrial mammals is quite in 

 accordance with what Mr. Darwin has shown is the rule in oceanic 

 islands ; and, moreover, the argument in favour of gradual coloniza- 

 tion also supports that of want of continuity of land-surface. 



4. The colonists came from the Palaarctic Region. 

 This is capable of consideration under two headings : — 1, the Palae- 



1 There seems reason to believe that many genera of insects known as Miocene 

 are also of Eocene age. 



2 L. c. p. 348. 



