88 THE ORIGIN OF VERTEBRATES 
the dominant race was neither crustacean nor arachnid, but partook 
of the characters of both; also, as is characteristic of dominance, 
there was very great variety of form, so that it seems more probable 
than not that some of these forms may have combined the arachnid 
and crustacean characteristics to the extent of possessing lateral eyes 
with an inverted yet compound retina. A certain amount of 
evidence points in this direction. As already stated, the compound 
retina which characterizes the vertebrate lateral eyes is character- 
istic of all facetted eyes, and in the trilobites facetted lateral eyes 
are commonly found. From this it may be concluded that many of 
the trilobites possessed eyes with a compound retina. There have, 
however, been found in certain species, e.g. Harpes vittatus and 
Harpes ungula, lateral eyes which were not facetted, and are believed 
by Korschelt and Heider to be of an arachnid nature. They say, 
“Paleontologists have appropriately described them as ocelli, 
although, from a zoological point of view, they do not deserve this 
name, having most probably arisen in a way similar to that con- 
jectured in connection with the lateral eyes of scorpions.” If this 
conjecture is right, then in these forms the retina may have been 
inverted, but because they belonged to the trilobite group, the retina 
was most probably compound, so that here we may have had the 
combination of the arachnid and crustacean characteristics. On the 
other hand, in some forms of Branchipus, and many of the Gamma- 
ride, a single corneal lens is found in conjunction with an eye of the 
crustacean type, so that a non-facetted lateral eye, found in a fossil 
form, would not necessarily imply the arachnid type of eye with the 
possibility of an inverted retina. Whatever may be the ultimate 
decision upon these particular forms, the striking fact remains, that 
both in the vertebrate and in the arachnid the median eyes possess 
a simple upright retina, while the lateral eyes possess an inverted 
retina, and that both in the vertebrate and the crustacean the 
median eyes possess a simple upright retina, while the lateral eyes 
possess a compound retina. 
The resemblance of the retina of the lateral eyes of vertebrates 
to that of the lateral eyes of many arthropods, especially crustaceans, 
has been pointed out by nearly every one who has worked at these 
invertebrate lateral eyes. The foundation of our knowledge of the 
compound retina is Berger’s well-known paper, the results of which 
are summed up by him in the following two main conclusions. 
