2 8o 



NATURE 



[July iS, 1895 



(f) land plants. For convenience of reference, our present 

 knowledge of the time-range of these kinds may be presented in 

 tabular form. The accompanjnng illustration, representing the 

 whole of geological time by its height, indicates in a general i 

 way by jxrr]ientlicular lines the time-range of the kinds just 

 mentioned, and remarks in following paragraphs further explain 

 the known range of some of the subordinate, as well as that of 

 the principal kinds. 



The horizontal spaces of the table represent the systems or 

 stages of the geological scale. The |iro|X)rtionate width of the 

 spaces which contain the names of those systems or stages is not 

 intended to indicate the actual ratio of geolc^cal time for each, 

 but it may be stated as the general opinion of comix-tenl 

 nvestigalors that the portion of the scale from the Cambrian 

 ' the Carboniferous inclusive represents a much greater length 

 of time than does the portion from the Trias to the Tertiar)- 

 inclusive. In other words, it is generally believed that the 

 PaktMzoic portion of the geological scale was of much longer 

 duration than was that of the Mesozoic and Cenozoic ]X)rtions 

 together. 



The perpendicular lines in the table, which are placed singly 

 or ill jsiirs or groups under letter; of the alphabet from .V to G 



CARBONIFEROU! 



CRETACEOUS 



U. SILURIAN 



L. SItURIAM 



Time r.T 

 inv- 



mojiii 



M.nrinc invertebrates, (tt) non.ni.irinc niid land 

 . (I)) batrachian*! and reptiles, (|0 liirds, (i--) 

 1 iiiants. 



inclusive, represent the time-range of the kinds of animals and 

 plants which have already been mentioned, and which for 

 convenience of reference arc again recorded with their corre- 

 .sixjniling letters at the foot of the table. This method of 

 grouping the different kinds of animals and plants, as alrcatly 

 ir' .11 AcA only for present convenience in making 

 r r.inological values. .Ml the princi|)al kinds 



V id in the usual systematic classification are. 



: in these sjiccial groups, the few that are 

 -.irded as of little or no importance in this 

 coiiin. !i:)ji. I lie dotted (xirtion of certain of the lines indicates 

 uncertainty as to the real extent of the time-range which, is 

 ^t ' ■' ' .if iin|jerfect or doubtful represenlalion 



('! ' red fossil remains. 



•■ liive existed U|x)n the earth, .ind of 

 " . ered, only those of m.'irine inverte- 



■He through the whole geological 

 '- im|Kirtant |nrtions of the animal 

 gr<iup of five |)er|X!n(licular lines 

 1 '1. marine invertebrate life thus retire 



I.I. 



.Kale. 1 Ik 

 kingdom is r 

 under the Vu 



sented includes the Protozoa, Coelenterata, -Annuloida, Anmilosa, 

 and Mollusca, the latter including the MoUuscoida. That is, it 

 includes five of the six sub-kingdonts or branches of the animal 

 kingdom. 



The non-marine and land invertebrates, the tinie-i^ange of which 

 is intended to be represented in the table by the two perjiendicuiar 

 lines under the letter B, are only insects and fresh-water, 

 brackish-water, and land molluscs. The discovered fossil 

 remains of all other non-marine and hnd invertebrates are 

 regarded as either too rare or too imimportaiU to be profitably 

 considered in the comparisons which arc to f<illow. The longer 

 of the two lines may be taken as representing the known time- 

 range of insects, and the shorter that of land and non-marine 

 mollusca. 



The pair of peri>endicular lines in the table under the letter C 

 shows the apiiroximate time-range of all the various kinds of 

 animal remains which have been referred to the fishes. The 

 shorter of the two lines indicates the known range of the teleostcan 

 fishes, and the longer that of the other kinds, the latter 

 including certain forms that difier materially from any living 

 fishes. 



The time-range of batrachians and reptiles, so far as it is 

 known, is shown by the three perpendicular lines in the table 

 under the letter D, that of the dinosaurs alone l>eing represented 

 by the shortest line of the three. 



The known time-range of birds is represented by the single line 

 under the letter K. It is here assumed that most, if not all, the 

 fossil tracks found in Triassic strata, and formerly referred to 

 birds, are those of ilinosaurs. 



The two lines in the table under the letter T represent the 

 known time-range of mammals, the longer line representing 

 that of the non-placental, and the shorter that of the placental 

 mammals. 



The known time-range of land plants is represented by the two 

 lines under the letter I'l. The shorter line represents the range 

 of the dicotyledons and jxtlms, and the longer one that of all 

 other kinds. The alga' and diatoms are omitted from the table, 

 its being of little or no importance in the comparisons and dis- 

 cussions w hich are to follow. 



The earlier portion of the time-range for each of the kinds of 

 animals and plants, .is shown by Ihe per|ieiulicular lines in the 

 table, is naturally more incompletely and indefinitely represented 

 by fossil remains than is the later portion, Ijecause of the smaller 

 variety and greater rarity of those earlier remains, and also in 

 most cases because of the increasing difference in character from 

 living forms which is observable from later to earlier formations. 

 In some ca,ses, however, the early portion of the lime-range as 

 it is now known begins so suddenly, and with forms of such high 

 biological rank, as to make it evident that its real beginning w.as 

 much earlier than it has yet been proved to be by actual dis- 

 covery of fossil remains. The last-mentioned fact is of great 

 importance in many respects, but it does not necessarily aflect 

 the question mider consideration, because all estimates of the 

 relative chronological value of fossil remains must lie confined to 

 the kinds already known, and the apjilication of such estimates 

 nuist refer only to those portions of the geological scale in the 

 strata pertaining to which the remains are known to occur. 



It has liecn shown that it is the general advancement 

 in biological rank for all organic forms ami for the 

 whole of geological time that constitutes the ideal uliimate 

 standard of measure for that time. It does not necessiirily 

 follow, however, that the geological scale is actually liased upon 

 the combine<l average rate of advancement of all those forms, 

 because this is a factor which cannot be definitely ascertainetl. 

 .Still, in all cases it is necessary to apply that idea .so far as is 

 pr.acticable. 



In view of the facts recorded in the preceding paragraphs, 

 Ihe highest estimate of chronological value must necessarily be 

 placed nixjn the fossil remains of those kinds which have existed 

 under the most nearly uniform conditions through the whole of 

 geological time, and which give evidence of the most nearly 

 uniform advancement in biological rank. Accordingly, the 

 remains of marine inverlel)rates jxissess legitimate claims to a 

 higher estimate of chronological value than do those of any other 

 kinds of animals or of plants. 



It is true that the rate of development in biological rank of 

 marine invertebrates does not embrace the entire advance for the 

 whole animal kingdom, liecause it begins in the scale as il is 

 now known with many highly organised forms, and endswilhoul 

 including the vertebrates ; but this fact does not affect any of the 



NO. 1342, VOL. 52] 



