GRAND DISCOVERIES OF LIFE 
9 
spiders. Because of the fact that this was the largest and most 
highly specialized group of invertebrates zoologists were singularly 
unanimous in opinion that it was too specialized possibly to give 
rise to any new types. 
When working upon the development of the eyes of arthropods 
Patten found that the fore-brain of an embryo scorpion was 
gradually covered by an overgrowing fold of skin that converted 
the brain into a hollow vesicle. During the process one or two 
pairs of eyes were transferred from the outer surface of the head 
to the blind end of a median tube that projected from the mem¬ 
braneous roof of the brain. This procedure was unique among 
invertebrates; but it was so exact a counterpart of what took 
place in the formation of a rudimentary pineal eye of vertebrates 
as to indicate some intimate genetic relationship 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 ver¬ 
tebrates in its general shape, in its subdivision into several regions, 
in the general nature of the functions performed by these re¬ 
gions, and in the character of their appropriate nerves, ganglia 
and sense organs; that the arachnids possessed 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.” 
Now it was hardly possible that vertebrates could by any route 
come from modern air-breathing scorpions or spiders for the 
lowest vertebrates undoubtedly came from marine animals. 
Modern land arachnids were manifestly decendants of a large 
group of very ancient marine arthropods, the trilobites and 
merostomes, or giant sea-scorpions, which flourished in the remote 
Cambric period, long before any vertebrates were known to 
exist. In greatly reduced numbers they continued through the 
following two geological periods, and occurred frequently in the 
very same deposits in which the first vertebrates appeared. 
At this point of the inquiry Paleontology stepped in to the rescue 
of Biology. In intimate association with the declining arachnids 
