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previous parents ; the lingida hiatus suggests a similar abandon- 

 ment and return of allied species in remote geological epochs. 

 In the case of Calijmene Blumenhachii, which extends from the 

 lower Silurian up to the Devonian, there is a great variety of 

 forms acknowledged to be within the limits of one species, dis- 

 playing a progressive variation amounting almost to specific dif- 

 ference. It may also be a question whether the geological hori- 

 zon of animal origin has yet been reached in our investigations. 

 He inquired of Prof. Agassiz if any vertebrate had ever been 

 found in strata lower than the upper Silurian. 



Prof. Agassiz remarked, as to these alleged migrations, that 

 we know that species are well circumscribed within the limits of 

 faunas ; and that before such a line of argument can be followed, 

 it must be shown that any species pass from one continent to an- 

 other, except from man's agency. In regard to the geological 

 horizon of animal origin, he observed that the azoic system of 

 rocks is not so metamorphosed as not to show traces of fossils if 

 they had existed ; fragments at least would be found ; yet these 

 rocks immediately underlie the Silurian strata rich in fossils. He 

 thought that in this lowest system of fossils there was such a co- 

 ordination of the animal series as shows that all its great and 

 principal classes were then existing. Pander has maintained the 

 existence of fishes below the point alluded to by Prof. Rogers, 

 from what are considered their remains ; he did not distrust 

 these conclusions of Pander, though many do. Trilobites are 

 found in the lowest beds ; these are complicated animals, and be- 

 long among the highest crustaceans ; in the three other great 

 divisions of the animal kingdom it is not the lowest, but the high- 

 est representatives that are found ; the earliest fishes are among 

 the most perfect of their class, and have many reptilian charac- 

 ters ; the mollusks belong to the high cephalopods, and the cri- 

 noids rank high among the echinoderms. In late general works, 

 eleven or twelve subdivisions of the earth's crust are given ; 

 D'Orbigny makes twenty-seven, but he was prepared to show the 

 occurrence of at least forty-eight successive periods of change, 

 with characteristic fossils found neither below nor above their 

 respective beds ; the alleged identity of fossils in different strata 

 was only apparent, and would be found so on actual comparison 

 of specimens. 



