CHAPTER XIX. 

 THE OSTRACODERMS AND THE MARINE ARACHNIDS. 



In the preceding chapters, we have shown that there is essential agreement 

 in the structure and mode of growth of the corresponding systems of organs in 

 the vertebrates and arthropods. This agreement is so intricate and all-pervading 

 that it is intelligible only on the assumption that the arthropods represent the 

 ancient stock from which the vertebrates arose. 



But in spite of this underlying agreement in structure, there is a wide dif- 

 ference in outward appearance between any living arthropod and any living 

 vertebrate. To demonstrate a direct genetic relationship between them, we must 

 fill this apparent gap with real animals that are intermediate in character, or 

 account in some other way for the abrupt transition. 



In either case, we must determine what are the highest arthropods and what 

 are the lowest vertebrates, and when and how the transition from one to the other 

 took place. Unfortunately there is no a priori way of deciding, in a phylogenetic 

 sense, what is "high" and what is "low," until we have found out what are the 

 main lines of progressive evolution, and the directions in which they lead. In an 

 inquiry of this nature there is only one method that can be used, namely the 

 picture-puzzle method, whereby we aim through repeated trials to fit all the facts 

 into a complete and intelligible picture, knowing full well that there is but one way 

 for them to fit, and that when they do, we shall have an accurate picture of the 

 truth. 



If on consulting the geological record we find in the remote past some period 

 toward which the genetic lines in question converge, blending there with a group 

 of animals having some characteristics of each, then there will be a very strong 

 presumption that we have correctly identified the upper and lower ends of the 

 break in the series; that that class of animals was the connecting link between 

 them; and that the actual transition took place at, or before, that period. 



If on further examination it can be shown that the hypothetical connecting 

 link resembles in several different ways the upper end of the lower series and the 

 lower end of the upper one, and forms with them a continuous graded series, with 

 a tendency on one side of the connecting link to produce special structures, or 

 special methods of growth that are either anticipated or find fuller expression on 

 the other, then our previous assumption of genetic relationship attains thereby 

 the rank of actual demonstration, and it will not be shaken by any amount of 

 negative evidence, or by the threatened collapse of cherished convictions that the 

 picture was going to be something very different from what it actually turns out 

 to be. 



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