io6 



NATURE 



[September 24, 1896 



rooks. TliL' liiwesl' and simplest of Ihesu G)-miiomy.\a represent 

 the starting-point pf that scheme of organic evoKition which we 

 are considering l()-day. The higher order of Protozoan life, the 

 siibgrade Corticiita, contains three Phyla, no one of which is 

 available in the fossil state. They are, however, of great interest 

 and importance to us as showing that the Protozoan type assumes 

 a far higher organisation on its way to evolve the more advanced 

 grade of animal life. The first-formed of these latter are con- 

 tained in the two Phyla of the sub-grade Cielentera, the Porifera 

 or jiponges, anil the Nematophora or Corals, Sea-Anemones, 

 Hydrozoa and allied groups. Both of these Phyla are plcntifull)' 

 represented in the fossil state. It is considered certain that the 

 latter of these, the Nematophora, gave rise to the higher sub- 

 graiie, the Coelomata, or animals with a calom or body-cavity 

 smrounding the digestive tract. This latter includes all the 

 remaining species of animals in nine Phyla, five of which are 

 found fossil — the Echinoderma, Gephyrea, Mollusca, Appen- 

 diculata, and Vertebrala. 



Before proceeding further, I wish to lay emphasis on the 

 immense evolutionary history which must have been passed 

 through before the ancestor of one of the higher of these nine 

 Phyla came into being. Let us consider one or two examples, 

 since the establishment of this position is of the utmost import- 

 ance for our argument. First, consider the past history of the 

 Vertebrata — of the common ancestor of our Balanoglossus, 

 Tunicates, Amphioxus, Lampreys, Fishes, Dipnoi, Amphibia, 

 Reptiles, Birds, and Mammals. Although zoologists differ very 

 widely in their opinions as to the affinities of this ancestral form, 

 they all agree in maintaining that it did not arise direct from 

 the Nematophora in the lower sub-grade of Metazoa, but that it 

 was the |)roduct of a long history within the Ccelomate sub- 

 grade. The question as to which of the other Crelomate Phyla 

 it was associated with will form the subject of one of our dis- 

 cussions at this meeting ; and I will therefore say no more upon 

 this period of its evolution, except to point out that the very 

 question itself, " the ancestry of Vertebrates," only means a 

 relatively small part of the evolutionary history of the Vertebrate 

 ancestor within the Creloniate group. For when we have 

 decided the question of the other Coelomate Phylum or Phyla to 

 which the ancestral \'ertebrate belonged, there remains of course 

 the history of that Phylum or those Phyla earlier than the point 

 at which the \'erlebrate diverged, right back to the origin of the 

 Ccelomata ; while, beyond and below, the wide gulf between 

 this and the Cielentera had to be crossed, and then, probably 

 after a long history as a Cadenterate, the widest and most 

 significant of all the morphological intervals — that between the 

 lowest Metazoon and the highest Protozoon — was traversed. 

 But this was by no means all. There remains the history within 

 the higher Protozoan sub-grade, in the interval from this to 

 the lower, and within the lower sub-grade itself, until we finally 

 retrace our steps to the lowest and simplest forms. It is im- 

 possible to suppose that all this history of change can have been 

 otherwise than immensely prolonged ; for it will be shown below 

 that all the available evidence warrants the belief that the 

 changes during these earlier phases were at least as slow as tho.se 

 which occurred later. 



If we take the history of another of the higher Phyla, the 

 Appendiculata, we find that the evidence points in the same 

 direction. The common ancestor of our Rotifera, earthworms, 

 leeches, Peripatus, centipedes, insects, Crustacea, spiders and 

 scorpions, and forms allied to all these, is generally admitted to 

 have been Ch;vtopod-like, and probably arose in relation to the 

 beginnings of certain other Cojlomata Phyla, such as the 

 Gephyrea and perhaps Mollusca. At the origin of the Ccelo- 

 mate sub-grade, the common ancestor of all Coelomate Phyla is 

 reached, and its evolution has been already traced in the case of 

 the Vertebrata. 



What is likely to be the relation between the time required 

 for the evolution of the ancestor of a Crclomate Phylum and 

 that required for the evolution, which subsequently occurred, 

 within the Phylum itself? The answer to this question depends 

 mainly upon the rate of evolution in the lower parts of the 

 animal kingdom as compared with that in the higher. Contrary, 

 perhaps, to anticipation, we find that all the evidences of rapid 

 evolution are confined to the most advanced of the smaller 

 groups within the highest Phyla, and especially to the higher 

 classes of the Vertebrata. Such evidence as we have strongly 

 indicates the most remarkable persistence of the lower animal 

 types. Thus in the class Imperforata of the Reticularia 

 (Foraminifera) one of our existing genera [Saccamina) occurs in 



NO. 1404, VOL. 54] 



the Carboniferous strata, another (Troc/iammina)\n the Permian, 

 while a single new genus {Receptaculites) occurs in the Silurian 

 and Devonian. The evidence from the class Perforata is much 

 stronger, the existing genera Nodosaria, Denlaliita, Texliilaria, 

 Graiiimos/omiiiii, Valvulina, and Nummuliita all occurring in 

 the Carboniferous, together with the new genera Archiediscus (?) 

 and Fusuliiia. 



I omit reference to the much-disputed P"ozoon from the 

 Laurentian rocks far below the horizon, which for the purpose of 

 this address I am considering as the lowest fossiliferous stratum. 

 We are looking forward to the new light which will be thrown 

 upon this form in the communication of its veteran defender. 

 Sir William Dawson, whom we are all glad to welcome. 



Passing the Radiolaria, with delicate skeletons less suited for 

 fossilisation, and largely pelagic and therefore less likely to 

 reach the strata laid down along the fringes of the continental 

 areas, the next Phylum which is found in a fossil state is that of 

 the Porifera, including the sponges, and divided into twoclasses, 

 the Calcispongiiv and Silicospongia;. All hough the fossilisation 

 of sponges is in many cases very incomplete, distinctly recog- 

 nisable traces can be made out in a large number of strata. 

 From the.se we know that representatives of all the groups of 

 both classes (except the llalisarcidoe, which have no hard parts) 

 occurred in the Silurian, Devonian, and Carboniferous systems. 

 The whole Phylum is an example of long persistence with 

 extremely little change. And the same is true of the Nema- 

 tophora : new groups indeed come in, sometimes extremely rich 

 in species, such as the PaUvozoic Rugose corals and CJraptolites ; 

 but they existed side by side with representatives of existing 

 groups, and they are not in themselves primitive or ancestral. 

 A study of the immensely numerous fossil corals reveals no 

 advance in organisation, while researches into the structure ot 

 existing Alcyonaria and Hydrocorallina have led to the inter- 

 pretation of certain Paleozoic forms which were previously 

 obscure, and the conclusion that they find their place close beside 

 the living species. 



All available evidence points to the extreme slowness of pro- 

 gressive evolutionary changes in the Crelenterate Phyla, although 

 the Protozoa, if we may judge by the Reticularia (Foraminifera), 

 are even more conservative. 



When we consider later on the five Ca.'lomate Phyla which 

 occur fossil, we shall find that the progressive changes were 

 slower and indeed hardly appreciable in the two lower and 

 less complex Phyla, viz. the Echinoderma and (jephyrea, as 

 compared with the Mollusca, Appendiculata, and Vertebrata. 



Within these latter Phyla we have evidence for the evolution 

 of higher groups presenting a more or less marked .advance in 

 organisation. And not only is the rate of develoimient more 

 rapid in the highest Phyla of the animal kingdom, but it appears 

 to be most rapid when dealing with the highest animal tissue, 

 the central nervous system. The chief, and doubtless the most 

 significant, difference between the early Tertiary mammals and 

 those which succeeded them, between the Secondary and Tertiary 

 reptiles, between man and the mammals most nearly allied to 

 him, is a difference in the size of the brain. In all these cases 

 an enormous increase w this, the dominant tissue of the body, has 

 taken place in a time which, geologically speaking, is very brief. 

 When s|)eaking later on upon the evolution which has taken 

 place within the Phyla, further details upon this subject will be 

 given, although in this as in other cases, the time at our dis- 

 posal demands that the exposition of evidence unist largely yield 

 to an exposition of the conclusions which follow from its study. 

 And undoubtedly a study of all the available evidence points 

 very strongly to the conclusion that in the lower grade, sub- 

 grades, and Phyla of the animal kingdom, evolution has been 

 extremely slow as compared with that in the higher. We do 

 not know the reason. It may be that this remarkable per- 

 .sistence through the stratified series of deposits is due to an 

 innate fixity of constitution which has rigidly limited the power 

 of variation ; or, more probably perhaps, that the lower members 

 of the animal kingdom were, as they are now, more closely 

 confined to particular environments, with particular sets of con- 

 ditions, with which they had to cope, and, this being successfully 

 accomplished, natural selection has done little more than keep 

 up a standard of organisation which was sufficient for their needs ; 

 while the higher and more aggressive forms ranging over many 

 environments, and always prone to encounter new sets of con- 

 ditions, were compelled to undergo responsive changes or to 

 succumb. But whatever be the cause, the fact remains, and is 

 of the highest importance for our argument. When the ancestor 



