Trilobites ami Echinoderms 199 



are likewise very favourable subjects for phylogenetic studies. 

 So far as the known record can inform us, the trilobites are 

 exclusively Palaeozoic in distribution, but their course must have 

 begun long before that era, as is shown by the number of distinct 

 types among the genera of the lower Cambrian. The group reached 

 the acme of abundance and relative importance in the Cambrian and 

 Ordovician; then followed a long, slow decline, ending in complete 

 and final disappearance before the end of the Permian. The newly- 

 hatched and tiny trilobite larva, known as the protaspis, is very near 

 to the primitive larval form of all the Crustacea. By the aid of the 

 correlated ontogenetic stages and the succession of the adult forms 

 in the rocks, many phylogenetic series have been established and a 

 basis for the natural arrangement of the whole class has been laid. 



Very instructive series may also be observed among the Echino- 

 derms and, what is very rare, we are able in this sub-kingdom to 

 demonstrate the derivation of one class from another. Indeed, there 

 is much reason to believe that the extinct class Cystidea of the 

 Cambrian is the ancestral group, from which all the other Echino- 

 derms, star-fishes, brittle-stars, sea-urchins, feather-stars, etc., are 

 descended. 



The foregoing sketch of the palaeontological record is, of necessity, 

 extremely meagre, and does not represent even an outline of the 

 evidence, but merely a few illustrative examples, selected almost at 

 random from an immense body of material. However, it will perhaps 

 suffice to show that the geological record is not so hopelessly incom- 

 plete as Darwin believed it to be. Since The Origin of Species was 

 written, our knowledge of that record has been enormously extended 

 and we now possess, no complete volumes, it is true, but some 

 remarkably full and illuminating chapters. The main significance of 

 the whole lies in the fact, that just in proportion to the completeness 

 of the record is the unequivocal character of its testimony to the 

 truth of the evolutionary theory. 



The test of a true, as distinguished from a false, theory is the 

 manner in which newly discovered and unanticipated facts arrange 

 themselves under it. No more striking illustration of this can be 

 found than in the contrasted fates of Cuvier's theory and of that of 

 Darwin. Even before Cuvier's death his views had been undermined 

 and the progress of discovery soon laid them in irreparable ruin, 

 while the activity of half-a-century in many different lines of inquiry 

 has established the theory of evolution upon a foundation of ever 

 growing solidity. It is Darwin's imperishable glory that he prescribed 

 the lines along which all the biological sciences were to advance to 

 conquests not dreamed of when he wrote. 



