430 



NATURE 



{Sept. 2, 1880 



of the coronal system. But here Ihe comparison ceases, and, 

 although we can trace in the paliEontological development of such 

 types as Archteocidaris or Bothriocidaris modifications which 

 would lead us without great difficulty, on the one side to the 

 Cidaridre, and on the other to the EchinothuriDe and DiadematidLV 

 of the present day, we cannot fail to see most definite indications 

 in some of the structural features of the Pala;chinidas of charac- 

 teristics which we have been accustomed to associate with higher 

 groups. The minute tuberculation, for instance, of the Cly- 

 peastroids and Spatangoids, already existing in the Melonitidx, 

 the genital ring, and anal system, are quite as much Echinid 

 as Cidarid. The polyporus genera of the group represent to a 

 certain extent the polypori of the regular Echini, and the lapping 

 of the actinal plates of the Cidarida; and of the coronal plates 

 in some of the Diadematida;, as well as the existence of such 

 CTenera as Tetracidaris, of four interambulacral plates in 

 Astropyga, and of a large number of ambulacral plates in >ome 

 of the recent Echinometrad.-e, all these are Pala;chinid characters 

 which we can explain on the theory of the independent develop- 

 ment of the structural features of which they are modifications. 

 We should, however, remember, that the existence of a large 

 number of coronal plates, especially interambulacral plates, in 

 the Pala;chinidae, is a mere vegetative character, which they hold 

 in common with all the Crinoids, — a character which is reduced 

 to a minimum among the Holothurians, and still persists in full 

 force among the Pentacrini of the present day, as well as the 

 Astrophytida; and Echinidne. 



It would lead me too far to institute the same comparison be- 

 tween the embryonic stages of the different orders of Echino- 

 derms and their earliest fossil representatives. We may, 

 however, in a very general way, state that we know the earliest 

 embryonic stages of the order of Echinoderms of to-day, which, 

 with the exception of the Blastoidea and Cystideans, are 

 identical with the fossil orders, and that as far as we know tliey 

 all begin at a stage where it would be impossible to disti)i;:^iish 

 a Sea-urchin from a Star-fish, or an Ophiuran, or a Crinoid, or 

 an Holothurian, — a stage in which the test, calyx, abactinal and 

 ambulacral systems are reduced to a minimum. From this 

 identical origin there is developed at the present day, in a 

 comparatively short period of time, either a .Star-fish, a Sea- 

 urchin, or a Crinoid ; and if we have been able successfully to 

 compare, in the development of typical structures, the embryonic 

 stages of the young Echini with their development in the fo sil 

 genera, we may fairly assume that the same process is applicable 

 when instituting the comparison within tlie different limits of 

 the orders, but with the same restrictions. That is, if we wish 

 to form some idea of the probable course of transformations 

 which the earliest Echinoderms have undergone to lead us to 

 those of the present day, we are justified in seeking for our 

 earliest representatives of the orders such Echinoderms as 

 resemble the early stages of onr embi-yos, and in followin'j, for 

 them as for the Echini, the modifications of typical structures. 

 These we shall have every reason to expect to find repeated in 

 the fossils of later periods, and, going back a step further we 

 may perhaps get an indefinite glimpse of that first Echinodermal 

 stage which should combine the structural features common to 

 all the earliest stages of our Echinoderm embryos. 



And yet, among the fossil Echinoderms of the oldest periods, 

 we have not as yet discovered this earliest type from which we 

 could derive either the Star-fishes, Ophiurans, Sea-urchins, or 

 Holothurians. With the exception of tlie latter, vv'hich we can 

 leave out of the question at present, we find all the orders of 

 Echinoderms appearing at the same time. Bnt while this is tlie 

 case, one of the groups attained in these earliest days a promin- 

 ence which it gradually loses with the corresponding development 

 of the Star-fishes, Opliinrans, and Sea-urchins, it has steadily 

 declined in importance ; it is a type of Crinoids, the Cystideans 

 which culminated during Palaeozoic times, and completely dis- 

 appeared long before the present day. If we compare the early 

 types of Cystideans to the typical embryonic Echinodermal type 

 of the present day, we find they have a general resemblance, and 

 that the Cystideans and Blastoids represent among the fossil 

 Echinoderms tlie nearest approach we have yet discovered to 

 this imaginary prototype of Echinoderms. 



This may not seem a very satisfactory result to have attained. 

 It certainly has been shown to be an impossibility to trace in the 

 palajontological succession of the Echini anything like a sequence 

 of genera. No direct filiation can be shown to exist, and yet 

 the very existence of persistent types, not only among Echino- 

 derms, but in every group of marine animals, genera which have 



continued to exist without interraption from the earliest epochs 

 at which they occur to the present day, would prove conclusively 

 that at any rate some groups among the marine animals of the 

 present day are the direct descendants of those of the earliest 

 geological periods. When we come to types which have not 

 continued as long, but yet which have extended through two or 

 three great periods, we must likewise accord to their latest 

 representatives a direct descent from the older. The very fact 

 that the ocean basins date back to the earliest geological periods, 

 and have afforded to the marine animals the conditions most 

 favourable to an unbroken continuity under slightly varying 

 circumstances, probably accounts for the great range in time 

 during which many genera of Echini have existed. If we 

 examine the interlacing in the succession of the genera charac- 

 teristic of later geological epochs, we find it an impossiljility 

 to deny their continuity from the time of the Lias to the 

 present day. The Cidaris of the Lias and the Rhabdoci- 

 daris of the Jura are the ancestors of the Cidaris of to-day. 

 The Saleniae of the loner Chalk are those of the Salenicc of to- 

 day. Acrosalenia extends from the Lias to the lower Cretaceous, 

 with a number of recent genera, « hich begin at the Eocene. 

 The Pygaster of to-day dates back to the Lias ; Echinocyamus 

 and Fibularia commence with the Chalk. Pyrina extends from 

 the lower Jura through the Eocene. The Echinobrissus of to- 

 day dates back to the Jura. Holaster lived from the lower 

 Chalk to the Miocene, and the Hemiaster of to-day cannot be 

 distingiiished from the Hemiaster of the lower Cretaceous. 



Such descent we can trace, and trace as confidently as we 

 trace a part of the population of North America of to-day as 

 the descendants of some portion of the population of the 

 beginning of this century. [But we can go no further with 

 confidence, and bold indeed would he be who would attempt 

 even in a single State to trace the genealogy of the inhabitants 

 from those of ten years before. We had better acknowledge 

 our inability to go beyond a certain point ; anytliing beyond 

 the general parallelism I have attempted to trace, which in no 

 way invalidates the other proposition, we must recognise as 

 hopeless. 



But in spite of the limits which have been assigned to this 

 general parallelism, it still remains an all-essential factor in 

 elucidating the history of pala;ontological development, and its 

 importance has but recently been fully appreciated. For, while 

 the fossil remains may give us a strong., presumptive evidence of 

 the gradual passage of one type to another, we can only imagine 

 this modification to take place by a process similar to that which 

 brings about the modifications due to different stages of growth, 

 — the former taking place in what may practically be considered 

 as infinite time when compared to the short life history wliich has 

 given us as it were a resume' ol the patoontological development. 

 We may well pause to reflect that in the two modes of develop- 

 ment we find the same periods of rapid modifications occurring 

 at certain stages of growth or of historic development, repeating 

 in a different direction the same phases. Does it then pass the 

 limits of analog)' to assume that the changes we see taking place 

 under our own eyes in a comparatively short space of time, — 

 changes which extend from stages representing perhaps the 

 original type of the group to their most complicated structures, 

 — may, perhaps, in the larger field of paLxontological develop- 

 ment, not have required the infinite time we are in the habit of 

 asking for them ? 



Paleontologists have not tbeen slow in following out" this 

 suggestive track, and those' who have been anatomists and 

 embryologists besides have not only entered into most interesting 

 speculations regarding the origin of certain gi'oups, but they 

 have carried on the process still further, and have given us 

 genealogical trees where we may, in the twigs and branches and 

 main limbs and trunk, trace the complete filiation of a group as 

 we know it today, and as it must theoretically have existed at 

 various times to its very beginning. While we cannot but admire 

 the boldness and ingenuity of these speculations upon genetic 

 connection so recklessly launched during the last fifteen years, 

 we find that with but few exceptions there is little to recommend 

 in reconstructions which shoot so wide of the facts as far as they 

 are known, and seem so readily to ignore them. The moment 

 we leave out of sight the actual succession of the fossils and the 

 ascertainable facts of postembryonic development, to reconstruct 

 our genealogy, we are building in the air. Ordinarily, the twigs 

 of any genealogical tree have only a semblance of truth ; they 

 lead us to branchlets having but a slight trace of probability, to 

 branches where the imagination plays an important part, to main 



