A Suggestion on Extinction. 473 



Turning to the Crustacea, we find forms belonging to the 

 Branchiopoda ranging with great persistence, and with 

 very small amount of change. Estheria is found from the 

 Devonian onwards, and Afus dates back with little variation 

 to the Trias. The Branchiopoda, as a group, have great 

 powers of resistance to their surroundings, and the ova 

 exceptionally so. The small amount of variation might be 

 put down to the habit of prolonged periods of partheno- 

 genesis, so frequent in the order ; but, on the other hand, it 

 seems that the great stability of character in the group may 

 denote great powers of clinging to an unspecialised condition, 

 in which cross-fertilisation would only be required at long 

 intervals, to prevent the ill effects produced by long-continued 

 reproduction of an asexual type. 



Ostracoda abound in all the deposits from the Lower 

 Palaeozoic onwards. The genera and species tend to 

 have very long ranges in time. Bairdia, Cytherella, and 

 Cypridina occur from the Ordovician to the present day 

 with very small amount of variation. The Cirripedia 

 show a slowly-increasing specialisation from the Lower 

 Palaeozoic onwards, and are never abundant as fossils. 



The genus Nehalia and its allies are the few remaining 

 forms of a group that attained much greater specialisation dur- 

 ing Palaeozoic times. The many very peculiar and specialised 

 genera, which occur in the deposits from the Cambrian to the 

 Carboniferous, have a very wide distribution in space and 

 extremely limited range in time. Nebalia has been shown to 

 have the power of living in water which is sufficiently 

 poisonous to destroy most organisms. 



The few fossil forms of the Schizopoda which occur in the 

 Palaeozoic rocks are strikingly like their living representa- 

 tives, some of them, perhaps, with even more specialised 

 characteristics. They appear to have been a very persistent 

 group, with slowly-acquired specialisation. 



The Decapoda first appear in the Trias, and become in- 

 creasingly important, and acquire their greatest specialisation 

 and dominance at the present time. 



The Trilobites appear in the lowest Cambrian in an advanced 

 condition of specialisation, and quickly acquired their maxi- 



VOL. XIV. 2 I 



