290 KANSAS CITY REVIEW OF SCIENCE. 
strata. It tells us that we have a right to expect, in some way, the trace of any 
class of animals that has been an inhabitant of our planet. 
Ostracoids appeared with the worms, and with their minute, frail shells have 
existed through all geological periods. 
Let us now examine a single form of Crustacean through a long geological 
period, and see the progress or want of progress, which it may make in the scale 
of being. Taking one of the earliest and most common, in species and individ- 
uals and most widely disseminated, we will examine its history. ‘This is the 
Trilobite. It is seen in all the twelve distinct regions of the Silurian, represented 
by hundreds of thousands of well preserved specimens. It is found in the lowest 
beds of the Cambrian and abounds till the close of the Permian. There are 
about two thousand known species, but all are characterized by three lobes in 
the body—a buckler, also divided into three lobes and compound eyes. ‘They 
are equally well characterized by an absence of any /osstlzed organs of locomo- 
tion, though from their known relations to other Crustaceans they must have had 
them. 
These traits they retain in all the varied species throughout their long geo- 
logical life, ending with the highest strata of the carboniferous age; or covering 
nearly three fourths of the whole of the earth’s geological history. 
The Trilobites are not the lowest of Crustaceans, but of ‘‘ highly complex and 
specialized types, and remote from the embryonic stages of the group to which 
they belong.”** They border on the Tetradecapods or middle rank of the sub- 
kingdom, but in all this long period of existence they never became Tetradeca- 
pods, or, in short, anything but Trilobites. There was time enough, and change 
in the earth’s condition sufficient to have shown evolution, if it had been a law 
of nature. The Trilobites, according to Dana’s time ratios then existed 37, 500,- 
000 years without losing their characteristic forms. 
It is a cardinal principle of evolution that ‘‘the use of an organ developes 
it.’} The Trilobites in common with all other Crustaceans must have had organs 
of locomotion, and some faint impressions on sand are supposed to have 
been made by their feet; but these organs were so thin or foliaceous that they 
have never been found fossilized, and this, too, at a time when rain-drops were 
imprinted on the rocks, and are seen to-day so delicately outlined that even the 
direction of the wind is known that attended the shower. Now if use of legs 
develops them, surely during 37,000,000 of years or as many generations, they 
should have become sufficiently firm to have left their shelly structure, or at least 
the impress of their feet beside the rain-drops. 
The arm of the blacksmith is indeed stronger for its use, but a life-long 
hammering adds no new muscle to it, and leaves so slight a mark on the bone 
that the anatomist cannot enter the catecombs and select a humerus, and say, ‘‘This 
is from a blacksmith,”’ even though, as in Europe, father and son for many gen- 
erations follow that employment. 
*Dawson. 
;Thomsson. 
