A PROBLEM IN EVOLUTION 425 



primitive vertebrates. The ancestral arachnids were marine forms, 

 present in the oldest records we have ; they nourished in the Cambrian, 

 and were the highest type of animals in existence at that time. The 

 ostraeoderms flourished in the following, or Silurian, period and were 

 the highest type of their time. They had some points in common 

 with their predecessors, the marine arachnids, and also with the true 

 fishes that appeared in the next, or Devonian, period, and which were 

 likewise the highest type of their time. The inference is obvious, that 

 the marine arachnids, the ostraeoderms, and the fishes, represent three 

 successive stages in the evolution of the animal kingdom, just as in the 

 later periods the fishes, amphibia and mammals represent successive 

 stages in the evolution of the vertebrates. If this inference is correct, 

 then the whole creative period in the evolution of the vertebrate stock 

 should become an open book, because the materials, both living and 

 fossil, with which one can unravel the evolution of the arachnids, are 

 apparently abundant and accessible. 



This situation demanded careful investigation, for the issues at 

 stake were very great. In 1889, when this problem for the first time 

 assumed definite shape in my mind, it was apparently impossible to 

 obtain well-preserved ostraeoderms in this country, nor did the known 

 remains in any country appear likely to yield more than the superficial 

 details of their anatomy. 



We were thus compelled to wait on opportunity, meantime, during 

 the next ten or eleven years, giving our attention to the anatomy and 

 embryology of living arachnids and the lower vertebrates, convinced 

 that a careful study along these lines would ultimately yield definite 

 evidence, one way or the other. The results fully justified this con- 

 clusion, for the longer this problem was studied, the more convinc- 

 ingly did it appear that the differences between these two great di- 

 visions of the animal kingdom were largely superficial and could be 

 legitimately explained. The resemblances were fundamental; they 

 were found in unexpected places, in independent systems of organs, 

 and they ran through successive stages in the growth of those organs. 

 It was clear that no other group of invertebrates resembled the verte- 

 brates in such a variety of ways, or to the same extent, as did the 

 arachnids; and no one has claimed that the main facts upon which 

 these resemblances were based are not substantially as I have stated 

 them to be. 



"We had demonstrated, therefore, that the marine arachnids are to 

 be regarded as the most probable ancestors of the vertebrate stock. 



V. The Ostracoderms 



But in spite of all that, there still remained a wide gap between the 

 arachnids and the vertebrates, and to bridge that gap we had to find 



