September 24, 1S96] 



A A TURE 



509 



rate during Palaeozoic times. They represent a large and dom- 

 inant Class, presenting more of the characters of the common 

 ancestor than the other Classes ; but the latter had diverged and 

 had become distinct long before the earliest fossiliferous rocks : 

 for we find well-marked representatives of the Crustacea in 

 Cambrian, and of the .Arachnids in Silurian strata. The Trilo- 

 bites, moreover, appear in the Cambrian with many distinct and 

 very ditilcrent forms, contained in upwards of forty genera, so 

 that we are clearly very far from the origin of the group. 



Of the lower group of Crustacea, the Entomostraca, the Cirri- 

 pedes are represented by two genera in the .Silurian, the Ostra- 

 codes by four genera in the Cambrian and over twenty in the 

 Silurian : of these latter two genera, Cythere and Bairdia, 

 continue right through the fossiliferous series and exist at the 

 present day. Remains of I'hyllopods are more scanty, but can 

 be traced in the Devonian and Carboniferous rocks. The early 

 appearance of the Cirripedes is of especial interest, inasmuch as 

 ihe fixed condition ofthe.se forms in the mature state is certainly 

 not primitive, and yet, nevertheless, appears in the earliest 

 representatives. 



The higher group, the Malacostraca, are represented by many 

 genera of I'hyllocarida in the Silurian and Devonian, and two in 

 the Cambrian. These also afford a good ex.ample of the im- 

 perfertion of the record, inasmuch as no traces of the group are 

 to be found between the Carboniferous and our existing fauna in 

 which it is represented by the genus Nebalia. The Phyllocarida 

 are recognised a.s the ancestors of the higher Malacostraca, and 

 yet these bttler already existed — in small numbers, it is true — 

 side by side with the Phyllocarida in the Devonian. The evolu- 

 tion ot the one into the other must have been much earlier. 

 Here, .as in the Arthropoda, we have evidence of progressive 

 evolution among the highest groups of the Class, as we see in 

 the comparatively late development of the Brachyura as com- 

 pared witn the Macrura. We find no trace of the origin of the 

 Class, or of the larger groups into which it is divided, or, 

 ijideed, of the older- among the small groupings into families 

 and genera.' 



( If the .\rachnida, although some of the most wonderful 

 examples of persistent types are to be found in this class, but 

 little can be .said. Merely to state the bare fact that three kinds 

 of scorpioji are found in the Silurian, two Pedipalpi, eight 

 scorpions, and two spiders in the Carboniferous, is sufficient 

 to shoiv that the period computed by geologists must be im- 

 mensely extended to account for the development of this Class 

 alone, inasmuch as it existed in a highly specialised 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, however, pointed out to 

 me that the Silurian scorpions possessed heavier limbs than those 

 of existing species, and this is a point in favour 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 Pakeozoic 

 group, and reach their highest known development at their first 

 appearance in the Silurian ; since then they have done nothing 

 but di^ppear gradually, leaving the single genus Limulus, un- 

 modified since its first appearance in the Trias, to represent 

 them. It is impossible to find clearer evidence of the decline 

 rather than the rise of a group. No progressive development, 

 but a gradual or rapid extinction, and consequent 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 Nautilidx. All 

 these groups begin with many forms in the oldest fossiliferous 

 r<x:ks, 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 persistent types 

 and the groups to which they belong — a period so extended that 

 the interval between the oldest Palaeozoic rocks and the present 

 day su]iplies no measurable unit? 



But I am digressing from the Appendiculate 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 developed and flourishing in Palaeozoic 

 times. The only evidence of progressive evolution is in the 



1 F^r an account or the evolution of the Crustacea 

 Addresso to the Geological Society in 189s and 



\V(.oii-.v:ir.i. 



NO. 1404, VOL. 54] 



the Presidential 

 > by Dr. Henry 



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 Ch;vtopod-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 Crelomate Phyla as I 

 have done the Appendiculata. 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. Representatives 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 genera Conularia, Hyolithes, Pterotheca, &c. , 

 are true Pteropods, as they are supposed to be, they occur in the 

 Cambrian and Silurian strata, while the group of Gastropods 

 from which they almost certainly arose, the Bullidii;, are not 

 known before the Trias. Furthermore, the forms which are 

 clearly the oldest of the Pteropods — Limacina and Spiriales — 

 are not known before the beginning of Ihe Tertiary Period. 

 Either there is a mistake in the identification of the Palajozoic 

 fossils as Pteropods, or the record is even more incomplete than 

 usual, and the most specialised of all Molluscan groups had been 

 formed before the date of the earliest fossiliferous rocks. If this 

 should hereafter be disproved, there can be no doubt about the 

 early appearance of the Molluscan Cias.ses, and that it is the 

 irony of an incomplete record which places the Cephalopods and 

 ("rastropods in the Cambrian and the far more ancestral Chiton 

 no lower than the Silurian. Throughout the fossiliferous series 

 the older families of Gastropods and Lamellibranchs are followed 

 by numerous other families, which were doubtless derived from 

 them ; new and higher groups of Cephalopods were developed, 

 and, with the older groups, either persisted until the present 

 time or became extinct. But in .all this splitting up of the 

 Classes into groups of not widely different morphological value, 

 there is very little progressive inodification, and, taking such 

 changes in such a period as our unit for the determination of the 

 time which was necessary for the origin of the Classes from a 

 form like Chiton, we are led to the same conclusion as that 

 which followed from the consideration of the Appendiculata, 

 viz. that the fossiliferous series would have to be multiplied 

 several times in order to provide it. 



Of the Phylum Gephyrea, I will only mention the Brachiopods, 

 which are found in immense profusion in the early Paleozoic 

 rocks and which have occupied the subsequent time in becoming 

 less dominant and important. So far from helping us to clear up 

 the myster)' which surrounds the origin of the Class, the earliest 

 forms are quite as specialised as those living now, and, some of 

 them (Lingula Discina) even generically identical. The demand 

 for time to originate the group is quite as grasping as that of the 

 others we have been considering. 



All the Classes of Echinodenna, except the Ilolothurians, 

 which do not possess a structure favourable for fossilisation, are 

 found early in the Palaeozoic rocks, and many of them in the 

 Cambrian. Although these early forms are very different from 

 those which succeeded them in the later geological periods, they 

 do not possess a structure which can be recognised as in any way 

 primitive or ancestral. The Echinoderma are the most distinct 

 and separate of all the Ccelomate Phyla, and they were ap- 

 parently equally distinct and separate at the beginning of the 

 fossiliferous series. 



In concluding this imperfect attempt to deal with a very vast 

 subject in a very short time, I will remind you that we were led 

 to conclude that the evolution of the ancestor of each of the 

 higher animal Phyla, probably occupied a very long period, per- 

 haps as long as that required for the evolution which subse- 

 quently occurred within the Phylum. But the consideration of 

 the higher Phyla which occur fossil, except the Vertebrata, leads 

 to the irresistible conclusion that the whole period in which the 

 fossiliferous rocks were laid down must be multiplied several times 

 for this later history alone. The period thus obtained requires to 

 be again increased, and perhaps doubled, for the earlier history. 

 Ill the preparation of the latter part of this address I have 

 largely consulted Zittel's great work. I wish also to express my 

 thanks to my friend Prof. Lankester, whom I have consulted 

 on many of the details, as well as the general plan which has 

 been adopted. 



