678 



SCIENCE, 



[N. S. Vol. IV. No. 97. 



us some vague conception of tLe rate at 

 which the process travels in this division 

 of the Arthropoda. If we look upon de- 

 velopment as a series of paths which, by 

 successively uniting, at length meet in a 

 common point, then some conception of the 

 position of that distant center may be 

 gained by measuring the angle of diver- 

 gence and finding the number of unions 

 which occur in a given length. In this 

 case the amount of approximation and 

 union shown in the interval between the 

 Carboniferous period and the present day 

 is relatively so small that it would require 

 to be multiplied many times before we 

 could expect the lines to meet in the com- 

 mon point, the ancestor of insects, to say 

 nothing of the far more distant past in 

 which the Tracheate Arthropods met in 

 an ancestor presenting many resemblances 

 to Peripatus. But it must not be forgotten 

 that all this vast undefined period is re- 

 quired for the history of one of the two 

 grades of one of the three branches of the 

 whole Phylum. 



Turning now to the brief consideration of 

 the second grade of Arthropods, distin- 

 guished from the first grade by the absence 

 of antennae, the Trilobites are probably the 

 nearest approach to an ancestral form met 

 with in the fossil state. Now that the pos- 

 session of true antennse is certain, it is rea- 

 sonable to suppose that the Trilobites repre- 

 sent an early class of the Aceratous branch 

 which had not yet become Aceratous. 

 They are thus of the deepest interest in 

 helping us to understand the origin of the 

 antennaless branch, not by the ancestral ab- 

 sence, but by the loss of true antennae which 

 formerly existed in the group. But the Trilo- 

 bites did not themselves originate the other 

 classes, at any rate during Palaeozoic times. 

 They represent a large and dominant class, 

 presenting more of the characters of the 

 common ancestor than the other classes ; 

 but the latter had diverged and had be- 



come distinct long before the earliest fos- 

 siliferous rocks ; for we find well-marked 

 representatives of the Crustacea in Cam- 

 brian, and of the Arachnida in Silurian 

 strata. The Trilobites, moreover, appear in 

 the Cambrian with many distinct and very 

 different forms, contained in upwards of 

 forty genera, so that we are clearly very far 

 from the origin of the group. 



Of the lower group of Crustacea, the En- 

 tomostraca, the Cirripedes are represented 

 by two genera in the Silurian, the Ostra- 

 codes by four genera in the Cambrian and 

 over twenty in the Silurian ; of these latter 

 two genera, Cythere and Bairdia, continue 

 right through the fossil if erous series and ex- 

 ist at the present day. Remains of Phyllo- 

 pods are more scanty, but can be traced in 

 the Devonian and Carboniferous rocks. The 

 early a.ppearance of the Cirripedes is of 

 special interest, inasmuch as the fixed con- 

 dition of these forms in the mature state is 

 certainly not primitive, and yet, neverthe- 

 less, appears in the earliest representatives. 



The higher group, Malacostraca, are 

 represented by many genera of Phyllocarida 

 in the Silurian and Devonian, and two in 

 the Cambrian. These also afford a good 

 example of the imperfection of the record, 

 inasmuch as no traces of the group are to 

 be found between the Carboniferous and 

 our existing fauna in which it is repre- 

 sented by the genus Nebalia. The Phyllo- 

 carida are recognized as the ancestors of 

 the higher Malcostraca, and yet these latter 

 already existed — in small numbers, it is 

 true — side by side with the Phyllocarida 

 in the Devonian. The evolution of the one 

 into the other must have been much earlier. 

 Here, as in the Arthropoda, we have evi- 

 dence of progressive evolution among the 

 highest groups of the class, as we see in 

 the comparatively late development of the 

 Brachyura as compared with the Macrura. 

 We find no trace of the origin of the class, 

 or of the larger groups into which it is di- 



