A PROBLEM IN EVOLUTION 433 



is there known as the infundibulum and the saccus vasculosus; it was 

 completely shut off from the outside world by the closing of the brain 

 tube during the process of embryonic growth. (4) The present mouth 

 of the vertebrates is a new one ; it arose through the transformation of 

 the so-called " dorsal organ/' or cephalic navel, an organ of obscure 

 function, but one that is present in all arthropods near the place where 

 the new mouth of the vertebrates ultimately appears. In the arthro- 

 pods it may serve as an organ of attachment, and at certain stages of 

 development affords a temporary passageway into the alimentary canal. 

 In other words, the arachnid brain could not continue to grow in vol- 

 ume, and in the particular way in which it had been growing, without 

 closing up the old mouth, and without forcing the jaws farther and 

 farther apart, until they reached the opposite side of the head. Here 

 they converged toward the cephalic navel, which then became a perma- 

 nent opening and was utilized as a new mouth to take the place of the 

 old one that was being closed up. 



Again, in still other words, we are here dealing with a series of 

 interlocking, internal organic adjustments that ultimately reached a 

 condition of unstable equilibrium. A radical and comparatively rapid 

 readjustment then took place, which led to a new condition of great 

 stability. This situation constituted a great crisis in the evolution of 

 this phylum, perhaps the most momentous crisis in the history of 

 organic evolution. But these revolutionary events were brought about 

 in an intelligible way by the cumulative action of long-established 

 methods of growth, which can be traced through the arachnid stock up 

 to the point where the ensuing events appear inevitable. The actual 

 consummation of them marks the transition from the vertebrates to the 

 invertebrates, and the beginning of a new class of animals. From the 

 nature of the case, the transition must have taken place somewhat 

 rapidly, and it probably occurred at some time during, or before, the 

 Silurian period. The paired jaws of the adult ostracoderms are intel- 

 ligible only on the assumption that they represent one of the early 

 phylogenetic stages of this process. 



VII. The Kecord of Paleozoic Events in Modern 

 Vertebrate Embryos 



The general way in which this metamorphosis took place is still 

 recorded in the embryonic history of the vertebrates to the present day, 

 for we can readily observe, in many vertebrate embryos, the shutting 

 up of the old mouth within the brain chamber, the transfer of at least 

 three pairs of jaws to the opposite side of the head, and their union 

 around the new mouth. 



In the embryo of the frog, for example, three pairs of rudimentary 



VOL. LXXXII.-30. 



