HISTORICAL REVIEW. 



crustacean or arachnid merostomata of the same period, are fundamentally differ- 

 ent from the latter in every character which admits of detailed comparison; they 

 are to be regarded merely as an interesting example of mimetic resemblance be- 

 tween organisms of two different grades adapted to live in the same way and 

 under precisely similar conditions." 



Surely no one knows the precise "way" or the precise conditions under 

 which these forms lived, or any probable advantage to be gained by one mimicing 

 the other. It would certainly be very remarkable if many members of one class 

 should mimic those of another, when the two classes were as fundamentally unlike 

 as the arthropods and vertebrates are supposed to be. 



All the ostracoderms are said to "mimic" the eurypterids, because they 

 have a similar shape, similar cephalic appendages, shell covered orbits and mouth 

 parts, and a similar minute structure of the dermal armor. But such a resem- 

 blance is too intricate and far-reaching to be accounted for on the ground of 

 mimicry, or functional parallelism, or mere coincidence; it can only be explained 

 on the ground of genetic relationship. 



Chamberlin and Salisbury have taken a less conservative position on this 

 question. In their recent text-book of Geology, it is stated, Vol. II, page 482 : 



No more suggestive combination of ancient life is presented by the geological 

 record than that which is found in these supposed fresh water deposits. The 

 type was foreshadowed by the eurypterids and fishes, of fish-like forms, that 

 appeared in the closing stages of the Silurian, but the record of that time is too 

 imperfect to disclose its deeper significance. Even with the much superior 

 material of the Devonian period, the more profound significance is only just 

 beginning to be realized. The center of interest is a unique group of ostracoderms 

 w r hich were at first interpreted as placoderm fishes, and later classed with the 

 jawless fishes (agnatha, lampreys, etc.), but which seem now to be clearly proven 

 to be an entirely distinct class lying between the arthropods and vertebrates, and 

 having some of the characteristics of each, but not truly belonging to either. Their 

 supreme interest lies in the force they give to the suggestion that the vertebrates 

 sprang from the arthropods. 



