Trilobites and 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. 
