1 48 



SCIENCE. 



justified in seeking for our earliest representatives of the 

 orders such Echinoderms as resemble the early stages of 

 our embryos, and in following, 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 would derive either the Star-fishes, Ophiu- 

 rans. Sea-urchins, or Holothurians. With the exception of 

 the latter, which we can leave out of the question at present, 

 we find all the orders of Echinoderms appearing at the same 

 time. But while this is the case, one of the groups attained in 

 these earliest days a prominence which it gradually loses with 

 the corresponding development of the Star-fishes, Ophiurans, 

 and Sea-urchins, it has steadily declined in importance ; it is a 

 type of Crinoids, the Cystideans which culminated during 

 Paleozoic times, and completely disappeared long before 

 the present day, If we compare the early types of Cysti- 

 deans 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 the nearest approach we have yet discovered 

 to this imaginary prototype of Echinoderms. 



This may not seem a very satisfactory result to have at- 

 tained. It certainly has been shown to be an impossibility 

 to trace in the paleontological succession of the Echini any- 

 thing like a sequence of genera. No direct filiation can be 

 shown to exist, and yet the very existence of persistent 

 types, not only among Echinoderms, but in every group of 

 marine animals, genera which have continued to exist with- 

 out interruption 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 geo- 

 logical 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 geo- 

 logical periods, and have afforded to the marine animals the 

 conditions most favorable 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 succes- 

 sion of the genera characteristic of later geological epochs, 

 we find it an impossibility to deny their continuity from the 

 time of the Lias to the present day. The Cidarisof the Lias 

 and the Rhabdocidaris of the Jura are the ancestors of the 

 Cidaris of to-day. The Saleniae of the lower Chalk are those 

 of the Saleniae of to-day. Acrosalenia extends from the 

 Lias to the lower Cretaceous, with a number of recent 

 genera, which begin at the Eocene. The Pygaster of to-day 

 dates back to the Lias ; Echinocyamus and Fibularia com- 

 mence 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 distin- 

 guished 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 at- 

 tempt 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 bevond a certain point ; any- 

 thing beyond the general parallelism I have attempted to 

 trace, which in no way invalidates the other proposition, we 

 must recognize 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 paleontological development, 

 and its importance has but recently been fully appreciated. 

 For, while the fossil remains may give lis ;i strong presump- 

 tive evidence of the gradual passage of one type to anothei , 

 we can only imagine this modification to take place by a pro- 

 cess 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 which has given us as it were a 

 risume of the paleontological development. We may well 

 pause to reflect that in the two modes of development 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 analogy 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 per- 

 haps the original type of the group to their most complicated 

 structures, — may, perhaps, in the larger field of paleontolog- 

 ical development, not have required the infinite time we are 

 in the habit of asking for them ? 



Paleontologists have not been slow in following out the 

 suggestive track, and those who have been anatomists and 

 embryologists besides have not only entered into most in- 

 teresting speculations regarding the origin of certain 

 groups, 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 to-day, 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 connec- 

 tion so recklessly launched during the last fifteen years, we 

 find that with but few exceptions there is little to recom- 

 mend 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 post-embryonic de- 

 velopment, 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 limbs 

 where it is finally allowed full play, in order to solve with 

 the trunk, to the satisfaction of the writer at least, the riddle 

 of the origin of the group. It seems hardly credible that a 

 school which boasts for its very creed a belief in nothing 

 which is not warranted by common sense should descend 

 to such trifling. 



The time for genealogical trees is passed ; its futility can, 

 perhaps, best be shown by a simple calculation, which will 

 point out at a glance what these scientific arboriculturists 

 are attempting. Let us take, for instance, the ten most 

 characteristic features of Echini. The number of possible 

 combinations which can be produced from them is so great 

 that it would take no less than twenty years, at the rate of 

 one new combination a minute for ten hours a day, to pass 

 them in review. Remembering now that each one of these 

 points of structure is itself undergoing constant modifica- 

 tions, we may get some idea of the nature of the problem 

 we are attempting to solve, when seeking to trace the gene- 

 alogy as understood by the makers of genealogical trees. 

 On the other hand, in spite of the millions of possible com- 

 binations which these ten characters may assume when 

 affecting not simply a single combination, but all the com- 

 binations which might arise from their extending over 

 several hundred species, we yet find that the combinations 

 which actually exist — those which leave their traces as fos- 

 sils—fall immensely short of the possible number. We 

 have, as I have stated, not more than twenty-three hundred 

 species actually representing for the Echini the results of 

 these endless combinations. Is it astonishing, therefore, 

 that we should fail to discover the sequence of the genera, 

 even if the genera, as is so often the case, represent, as it 

 were, fixed embryonic stages of some Sea-urchin of the 

 present day ? In fact, docs not the very history of the fossils 

 themselves show that we cannot expect this ? Each fossil 

 species, during its development, must have passed through 

 stages analagous to those gone through by the Echini of 

 the present day. Each one of these stages at every moment 

 represents one of the possible combinations, and those 

 which arc actually preserved correspond only to the parti- 

 cular period and the special combination which any Sea- 

 urchin has reached. These stages are the true missing 

 links, which we can no more expect to find preserved than 

 we can expect to find a record of the actual embryonic de- 

 velopment of the species of the present day without direct 



