A PROBLEM IN EVOLUTION 421 



IV. Fikst Clues to a New Solution 



As long ago as 1889, while working on the development of the eyes 

 of arthropods, the author discovered that the forebrain of the embryo 

 scorpion is gradually covered by an overgrowing fold of skin that con- 

 verts the brain into a hollow vesicle. During this process, one or two 

 pairs of eyes are transferred from the outer surface of the head to the 

 blind end of a median tube that projects from the membranous roof of 

 the brain. 



The details of the whole process by which the eyes were transferred 

 from the outer surface of the head to the inside of the brain were 

 unique in the invertebrates, and so similar to what takes in the forma- 

 tion of the rudimentary pineal eye of vertebrates, that it clearly pointed 

 to some intimate genetic relation between the two groups. 



To test what at first sight appeared to be so improbable, a careful 

 study of the anatomy and development of several types of arachnids 

 was made, and, much to our astonishment, it was found that the brain 

 of the arachnids resembled that of the vertebrates in its general shape, 

 in its subdivision into several regions, in the general nature of the 

 functions performed by these regions, and in the character of their 

 appropriate nerves, ganglia and sense organs; that the arachnids pos- 

 sessed skeletal structures comparable, respectively, with the dermal 

 bones, cranium, gill-bars and notochord of vertebrates; and finally it 

 was seen that the development of the embryo and the formation of the 

 germ layers in the arachnids, not only harmonized with, but illuminated 

 the corresponding conditions in the vertebrates. 1 



It was evident that in their fundamental structure the arachnids 

 resembled the vertebrates more than did any other invertebrates; and 

 they resembled them in so many different ways that it became more 

 and more improbable that all these resemblances could be mere coinci- 

 dences, or could be reasonably accounted for as duplications of struc- 

 ture due to similar functions, or to environment, or to any conceivable 

 cause other than community of origin. Nevertheless, it was hardly 

 possible that the vertebrates came from modern air-breathing scorpions, 

 or spiders, for the lowest vertebrates undoubtedly came from marine 

 animals. 



But the modern land arachnids are descendants of a large group of 

 very ancient marine arachnids, the trilobites and merostomata, or 

 giant sea-scorpions, which flourished in the early Cambrian and Ordo- 

 vician periods, long before any vertebrates were known to exist. They 

 were also found, although in rapidly diminishing numbers, in the two 



1 For a fuller description of these conditions, too technical to be repeated 

 here, see ' ' The Evolution of the Vertebrates and their Kin, ' ' by W. Patten. 

 P. Blakiston & Co., Philadelphia. 



