418 The Limits of Organic Evolution, [ May, 
marked the shorter toes were little by little lifted from the ground 
and became of little or no use: In successive ages we find the 
shorter toes becoming smaller while the middle toe becomes 
larger. This line of specialization has continued until it has 
reached a limit. in the horse, which has lost all but one toe on 
each foot, and walks on the extreme tip of this toe. Now it is 
perfectly evident that a limit has been reached in this case. The 
horse may perhaps in the future lose the rudimentary splint bones 
which still remain, but he can not lose his last toe; and it is 
therefore impossible to conceive any further development of the 
_ horse in this direction. Now the same principle will apply to 
all other lines of specialization, although we may not always 
beable to see what this limit may be. Physical laws would of 
themselves set limits to every line of advance, even if there be 
no such limits determined by the organism itself. 
It is easy to find examples which will show that such has been 
the general history of groups in the past. Some have reached 
eee the extreme of their development in the distant past, and have 
ceased to advance or disappeared. Others seem even now to be 
at the summit of their advance, and others still are yet advancing. 
The line of development represented by the trilobites has com- 
~ pletely exhausted itself. It rapidly approached its limits even in 
5 the Silurian, and then began to dwindle away and has disappeared 
= Completely. The brachiopods had also at this time reached their 
_ point of highest specialization, and became a highly developed 
group even at this early age. Since then they have remaine 
-~ stationary as to their organization, having steadily decreased in 
ats numbers, and the few that are left show no advance over the Silu- 
= rian forms. The cephalopod mollusks gradually increased in 
complexity during the Paleozoic, and finally a limit of the shelled 
_ forms was reached in the ammonites of the Jurassic and Creta- 
= ceous. The culmination was followed by extinction. Meantime 
~ a second line of development began, that of the naked cephalo- 
_ pods, and this has gone on advancing until the present time. The 
_ decapod Crustacea represent a group which is even now near its 
_ culmination. From their first appearance in the Carboniferous 
there has been a tendency toward concentration of organs toward 
the head. As this specialization advanced the abdomen became 
‘smaller while the head region became larger. Finally in the 
crabs, which appeared in the Jurassic, everything was concen- 
~ 
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