September 24, 1896] 



NA TURE 



507 



of one of the higher Phyla was associated with the lower Phyla 

 of the Crelomate sub-grade, when further l)ack it passed through 

 a Ciielcnterate, a higher Protozoan, and finally a lower Protozoan 

 phase, we must believe that its evolution was probably very slow 

 as compared with the rale which it subsequently attained. But 

 this conclusion is of the utmost importance ; for the history con- 

 tained in the stratified rocks nowhere reveals to us the origin of 

 a Phylum. And this is not mere negative evidence, but positive 

 evidence of the most unmistakable character. All the five 

 Cielomate Phyla which occur fossil ajijiear low down in the 

 Palaeozoic rocks, in the Silurian or Cambrian strata, and they 

 are represented by forms h hich are very far from being primitive, 

 or, if primitive, are persistent types, such as Chiton, which are 

 now living. Thus \'ertebrata are represented by fishes, both 

 sharks and ganoids ; the Appendiculata by cockroaches, scorpions, 

 Limulids. Trilobites, and many Cnistacea ; the Mollusca by 

 Nautilus and numerous allied genera, by Dentalium, Chiton, 

 Pteropnds, and many Gastropods and Lamellibranchs ; the 

 Gephyrca by very numerous Brachiopods, and many Polyzoa ; the 

 Echinoderma by Crinoids, Cystoids, Blastoids, Asteroids, 0]ihiu- 

 roids, and Echinoids. It is just conceivable, although, as I 

 believe, most improbable, that the Vertebrate Phylum originated 

 at the time when the earliest known fossiliferous rocks were laid 

 down. It must be remembered, however, that an enormous 

 morphological interval separates the fishes which appear in the 

 Silurian .strata from the lower liranches, grades, and classes of 

 the Phylum in which Balanoglossus, the Ascidians, Amphioxus, 

 and the Lampreys are placed. The earliest Vertebrates to 

 appear are, in fact, very advanced members of the Phylum, and, 

 from the point of view of anatomy, much nearer to man than to 

 Amphioxus. If, however, we grant the imjiroliable contention 

 that so highly organised an animal as a shark could be evolved 

 from the ancestral vertebrate in the period which intervened 

 between the earliest Cambrian strata and the Upper Silurian, it 

 is quite impossible to urge the same with regard to the other 

 Phyla. It has been shown above that when these appear in the 

 Cambrian and Silurian, they are flourishing in full force, while 

 their numerous specialised forms are a positive proof of a long 

 antecedent history within the limits of the Phylum. 



If, however, we assume for a moment that the Phyla began in 

 the Cambrian, the geologist's estimate must still be increased 

 considerably, and perhaps doubled, in order to account for the 

 evolutii in of the higher Phyla from forms as low as many which are 

 now known upon the earth ; unless, indeed, it is supposed, against 

 the whole weight of all such eviilence as is available, that the 

 evolutionary history in these early times was comparatively rapid. 



To recapitulate, if we represent the history of animal evolution 

 by the form of a tree, we find that the following growth took 

 place in some age antecedent to the earliest fossil records, before 

 the establishment of the higher Phyla of the Animal Kingdom. 

 The main trunk, representing the lower Protozoa divided, 

 originating the higher Protozoa ; the latter portion again divided, 

 probably in a threefold manner, originating the two lowest 

 Melazoan Phyla, con.stituting the (icelentera. The branch repre- 

 senting the higher of these Phyla, the Nematophora, divided, 

 originating the lower Ccelomale Phyla, which again branched 

 and originated the higher Phyla. And, as has been shown 

 above, the relatively ancestral line, at every stage of this complex 

 history, after originating some higher line, itself continued down 

 to the present day, throughout the whole series of fossiliferous 

 rock.s, with but little change in its general characters, and prac- 

 tically nothing in the way of progressive evolution. Evidences 

 of marked advance are to be found alone in the most advanced 

 groups of the latest highest products — the Phyla formed by the 

 last of these divisions. 



It may he asked how is it possible for the zoologist to feel so 

 confident as to the pa.st history of the various animal groups? I 

 have already explained that he does not feel this confidence as 

 regards the details of the histoyr, but as to its general lines. The 

 evidence which leads to this conviction is based upon the fact 

 that animal structure and mode of develojiment can be, and have 

 been, handed down from generation to generation from a period 

 far more remote than that which is represented by the earliest 

 fossils ; that fundamental facts in structure and development may 

 remain changeless amid endless changes of a more general 

 character ; that especially favourable conditions have preservetl 

 ancestral forms comparatively unchanged. Working upon this 

 material, comparative anatomy and embryology can recon.struct 

 for us the general aspects of a history which took place long 



NO. 1404, VOL. 54] 



before the Cambrian rocks were depo.sited. This line of reason- 

 ing may appear very speculative and unsound, and it may easily 

 become so when pressed too far. But applied with due caution 

 and reserve, it may be trusted to supply us with an immense 

 amount of valuable informati(m which cannot be obtained in any 

 other way. Furthermore, it is capable of standing the very true 

 and searching test supplied l>y the verification of predictions 

 made on its authority. Many facts taken together lead the 

 zoologist to believe that A was descended from C through B : 

 but if this be true, B should possess certain characters which are 

 not known to belong to it. Under the inspiration of hypothesis 

 a more searching investigation is made, and the characters are 

 found. Again, that relatively small amount of the whole scheme 

 of animal evolution which is contained in the fossiliferous rocks 

 has furni.shed abundant confirmation of the validity of the zoo- 

 logist's method. The comparative anatomy of the higher Verte- 

 brate Classes leads the zoologist to believe that the toothless 

 beak and the fused caudal vertebrte of a bird were not ancestral 

 characters, but were at some time derived from a condition more 

 comfortable to the general plan of vertebrate construction, and 

 especially to that of reptiles. Numerous secondary fossils prove 

 to us that the birds of that time possessed teeth and separate 

 caudal vertebn-e, culminating in the long lizard-like tail of 

 Archffiopteryx. 



Prediction and confirmation of this Icind, both zoological and 

 palajontological, have been going on ever since the historic point 

 of view was adopted by the naturalist as the outcome of Darwin's 

 teaching, and the zoologist may safely claim that his method, con- 

 firmed by palaeontology so far as evidence is available, may be 

 extended beyond the period in which such evidence is to be found. 



And now our last endeavour must be to obtain some conception 

 of the amount of evolution which has taken place within the 

 higher Phyla of the Animal Kingdom during the period in which 

 the fossiliferous rocks were deposited. The evidence must 

 necessarily be considered very briefly, and we shall be compelled 

 to omit the Vertebrata altogether. 



The Phylum Appendiculata is divided by Lankester into three 

 branches, the first containing the Rotifera, the .second the 

 ChLVtopoda, the third the Arthropoda. Of these the second is 

 the oldest, and gave rise to the other two, or, at any rate, to the 

 Arthropoda, with which we are alone concerned, inasmuch as the 

 fossil records of the others are insufficient. The Arthropoda 

 contain .seven Classes, divided into two grades, according to the 

 presence or absence of antennse — the Ceratophora, containing 

 the Peripatoidea, the Myriapoda, and the Hexapoda (or insects) ; 

 the Acerata, containing the Crustacea, Arichnida, and tw^o other 

 classes (the Pantopoda and Tardigrada) which we need not con- 

 sider. The first Class of the antenna-bearing group contains the 

 single genus Peripatus — one of the most interesting and ancestral 

 of animals, as proved by its structure and development, and by 

 its immense geographical range. Ever since the researches of 

 Moseley and Balfour, extended more recently by those of 

 Sedgwick, it has been recognised as one of the most beautiful of 

 the connecting links to be found amongst animals, uniting the 

 antenna-bearing Arthropods, of which it is the oldest member, 

 with the Ch;etopods. Peripatus is a magnificent example of the 

 for-reaching conclusions of zoology, and of its superiority to 

 paln;ontology as a guide in unravelling the tangled history of 

 animal evolution. Peripatus is alive to-day, and can be studied 

 in all the details of its .structure and development ; it is infinitely 

 more ancestral, and tells of a far more remote past than any 

 fossil Arthropod, although such fossils are well known in all the 

 older of the Palaeozoic rocks. And yet Peripatus is not known 

 as a fossil. Peripatus has come down, with but little change, 

 from a time, on a moderate estimate, at least twice as remote as 

 the earliest known Cambrian fossil. The agencies which, it is 

 believed, have crushed and heated the Archcean rocks so as to 

 obliterate the traces of life which they contained were powerless 

 to eflace this ancient type, for, although the passing generations 

 may have escaped record, the likeness of each was stamped on 

 that which succeeded it, and has continued down to the present 

 day. It is, of course, a perfectly trite and obvious conclusion, 

 but not the less one to be wondered at, that the force of heredity 

 should thus far outlast the ebb and flow of terrestrial change 

 throughout the vast period over which the geologist is our guide. 



If, however, the older Paleozoic rocks tell us nothing of the 

 origin of the antennre-bearing Arthropods, what do they tell us 

 of the history of the Myriapod and Hexapod Classes ? 



The Myriapods are well represented in Palaeozoic strata, two 

 species being found in the Devonian and no less than thirty-two 



