I46 THE PINEAL ORGAN 



Insecta, therefore, must have been derived from a primitive arthropod 

 stock in which a compound eye had not yet been developed, and con- 

 sidering that a fully formed Orthopteran insect is already found in Silurian 

 strata, this is what we might naturally expect. 



Intimately associated with the question of the origin of the compound 

 eyes in trilobites is that concerned with what are described as blind 

 trilobites, a problem which was investigated by Cowper Reed in 1898. 

 These he conceived as being divisible into two groups : 



(1) those in which the eyes were not present, because of the low 



phylogenetical and morphological rank of the genera in question, 

 as their general structure and stratigraphical appearance indi- 

 cate ; and 



(2) those which are genetically identical or closely allied to forms 



possessing eyes which are of high phylogenetic rank, and have 

 lost their visual organs by a secondary modification, presumably 

 as a result of adaptation to special conditions. 

 The first group he called the " primitive group " and the second the 

 " adaptive group." These will be described later in the section on 

 Geological Evidence of the Presence of Median Eyes in Extinct Animals, 

 Chapter 23. Quite apart from evidence of the existence of trilobites in 

 such ancient geological strata as the Upper Cambrian, the extreme 

 variability in the types of eye met with in trilobites (Figs. 97, 98, 99, 103) 

 as well as the marked differences in their general form point to the great 

 antiquity of these primitive arthropods and the probability not only of 

 evolutionary changes occurring in the direction of differentiation of more 

 complex eyes from simpler types, but also, as has been suggested by 

 Cowper and others, of degenerative or devolutionary changes taking place 

 in the eyes of certain species. The recognition of such degenerative 

 changes, as we shall see later, plays an important part in the interpretation 

 of the conditions which are found both in the median eyes of invertebrates 

 and in the pineal system of vertebrates. 



