A PROBLEM IN EVOLUTION 43* 



But " the labor we delight in physics pain," and patient labor is but 

 the measure of hoped-for rewards. In this case the realization ex- 

 ceeded all reasonable expectations; for, if he may be counted fortunate 

 who, by chance, finds some soothsaying relic of early life, how much 

 more than fortunate, how exceedingly blest, is the naturalist who is 

 permitted to lift with his own hands from some ancient storehouse of 

 the earth a long-songht-for treasure, and to awake in it, with the chisel's 

 kiss a spirit from the childhood era of the world ! 



VI. A Critical Period in Organic Evolution 



The net result of this fortunate find was to show that the ostraco- 

 derms, as had been predicted, were neither vertebrates nor invertebrates, 

 but a class intermediate between the two. They were, in fact, the real 

 missing links in the animal kingdom. The posterior part of the body 

 was membranous and decidedly fish-like in shape; but the contour of 

 the whole animal, especially the head, the nature of the appendages, 

 the eyes and the mode of locomotion, were more like those of the marine 

 scorpions. The gill, or atrial, chamber, and the structure of the 

 dermal skeleton were intermediate in character (Figs. 2 and 6). 



But the most important features of all were the long sought for 

 mouth parts, or jaws. They were paired, consisting of four separate 

 jaws, which in chewing, or biting, moved to and from the median line, 

 like the jaws of all known aithropods (Fig. 6, A). They were not 

 unpaired arches moving forward and backward, as they do in all true 

 vertebrates. 



To realize the significance of this fact, it must be understood that 

 one of the greatest differences between a vertebrate and an arachnid, 

 or arthropod, is the position and character of the jaws and mouth. In 

 all arthropods and arachnids, the mouth and jaws are primarily located 

 on the same side of the body as the nervous system ; food enters the 

 alimentary canal by a passage-way in the floor of the brain; and there 

 may be several pairs of jaw-like legs, which, in chewing, work in a 

 lateral direction to and from the median line. In the adult vertebrates 

 the jaws lie on the opposite side of the body from that on which the 

 brain and nerve cord are located ; the food that enters the mouth passes 

 directly into the alimentary canal without going through a passage-way 

 in the floor of the brain; and the jaws are two unpaired arches that 

 work against each other in a forward and backward direction (Fig. 7, 

 A and B). 



It is evident that either we are not dealing with the same things in 

 the two classes, or that there has been some change in their relative 

 locations. As a matter of fact, it is part one and part the other; for 

 we have been able to demonstrate: (1) That the nervous system of the 



