120 STUDIES IN EVOLUTION 



(1889): "We have endeavored to demonstrate that a natural 

 classification may be made by means of a system of analysis 

 in which the individual is the unit of comparison, because 

 its life in all its phases, morphological and physiological, 

 healthy or pathological, embryo, larva, adolescent, and old 

 (ontogeny), correlates vi^ith the morphological and physiolog- 

 ical history of the group to which it belongs (phjdogeny). " 

 It is also interesting to note that Agassiz^ recognized in 

 ontogeny a standard of classification. One of his strongest 

 statements is as follows : " Embryology [= ontogeny] will in 

 the end furnish us with the means of recognizing the true 

 affinities among all animals, and of ascertaining their relative 

 standing and normal position in their respective classes with 

 the vitmost degree of accuracy and precision." 



These principles can be best applied in a group of animals 

 which has a geological history more or less complete, and 

 which is not wholly parasitic or greatly degenerated. It is 

 of the greatest importance, also, to study the ontogeny of 

 primitive and non-specialized species, because without very 

 complete paleontological evidence the development of a much 

 later derived form may be so involved with larval adaptations 

 and accelerated characters as to be misleading. 



The trilobites lend themselves to this treatment in fulfil- 

 ling most of the necessary conditions. They have a known 

 geological history stretching through the entire Paleozoic, 

 from the beginning of the Cambrian to the Permian. Their 

 structure is generalized and quite uniform, and no sessile, 

 attached, parasitic, land, or freshwater species are known. 

 The ontogeny of all the principal groups has been studied, 

 including Cambrian, Ordovician, Silurian, and Devonian 

 types. 



The trilobites necessarily furnish little information of the 

 stages of growth which may be classed as embryonic. The 

 early embryonic stages are not preserved as fossils, and there- 

 fore may be omitted. In this category are the protemhryo, 

 or the ovum in its unsegmented and segmented stages (the 

 so-called " eggs of trilobites " may of course represent any 



