NOVEMBEE 6, 1896.] 



SCIENCE. 



679 



vided, or, indeed, of the older among the 

 small groupings into families and genera.* 



Of the Arachnida, although some of the 

 most wonderful examples of persistent 

 types are to be found in this class, but lit- 

 tle can be said. Merely to state the bare 

 fact that three kinds of scorpions are found 

 in the Silurian, two Pedipalpi, eight scor- 

 pions, and two spiders in the Carboniferous, 

 is sufficient to show that the period com- 

 puted by geologists must be immensely ex- 

 tended to account for the development of 

 this class alone, inasmuch as it existed in a 

 highly specialized condition almost at the 

 beginning of the fossiliferous series ; while, 

 as regards so extraordinarily complex an 

 animal as a scorpion, nothing apparent in 

 the way of progressive development has 

 happened since. Prof. Lankester has, how- 

 ever, pointed out to me that the Silurian 

 scorpions possessed heavier limbs than 

 those of existing species, and this is a point 

 in favor of their having been aquatic, 

 like their near relation, Limulus. If so, 

 it is probable that they possessed external 

 gills, not yet inverted to form the lung-book. 

 The Merostomata are, of course, a Palaeozoic 

 group, and reach their highest known de- 

 velopment at their first appearance in the 

 Silurian ; since then thej^ have done noth- 

 ing but disappear gradually, leaving the 

 single genus Limulus, unmodified since its 

 first appearance in the Trias, to represent 

 them. It is impossible to find clearer evi- 

 dence of the decline rather than the rise of 

 a group. No progressive development, but 

 a gradual or rapid extinction, and conse- 

 quent reduction in the number of genera 

 and species, is a summary of the record of 

 the fossiliferous rocks as regards this group 

 and many others, such as the Trilobites, 

 the Brachiopods and the Nautilidae. All 



* For an account of the evolution of the Crustacea 

 see the Presidential Addresses to the Geological So- 

 ciety of London in 1895 and 1896 by Dr. Henry 

 Woodward. 



these groups begin with many forms in the 

 oldest fossiliferous rocks, and three of them 

 have left genera practically unchanged from 

 their first appearance to the present day. 

 What must have been the time required to 

 carry through the vast amount of structural 

 change implied in the origin of these persist- 

 ent types and the groups to which they be- 

 long — a period so extended that the interval 

 between the oldest Palaeozoic rocks and the 

 present day supplies no measureable unit ? 



But I am digressing from the Appendicu- 

 late Phylum. We have seen that the fossil 

 record is unusually complete as regards two 

 classes in each grade of the Arthropod 

 branch, but that these classes were well de- 

 veloped and flourishing in Palaeozoic times. 

 The only evidence of progressive evolution 

 is in the development of the highest orders 

 and families of the classes. Of the origin of 

 the classes nothing is told, and we can 

 hardly escape the conclusion that for the 

 development of the Arthropod branches 

 from a common Chsetopod-like ancestor, and 

 for the further development of the classes 

 of each branch, a period many times the 

 length of the fossiliferous series is required, 

 judging from the insignificant amount of 

 development which has taken place during 

 the formation of this series. 



It is impossible to consider the other 

 Ccelomate Phyla as I have done the Ap- 

 pendiculata. I can only briefly state the 

 conclusions to which we are led. 



As regards the Molluscan Phylum, the 

 evidence is perhaps even stronger than in 

 the Appendiculata. Eepresentatives of 

 the whole of the classes are, it is believed, 

 found in the Cambrian or Lower Silurian. 

 The Pteropods are generally admitted to be 

 a recent modification of the Gastropods, 

 and yet, if the fossils described in the gen- 

 era Conularia, Hyolithes, Pterotheca, etc., 

 are true Pteropods, as they are supposed to 

 be, they occur in the Cambrian and Silu- 

 rian strata, while the group of Gastropods 



