The Principles of Palaeontology. 165 



The existence of such forms which can not be assigned to any 

 of the natural series, often causes a complication in the relations 

 we suppose to have existed among organisms in the course of time, 

 and often, too, explains the divergencies manifested in the views of 

 different authors. It brings in evidence a principle which at JGlrst 

 appears diametrically opposed to the principle of the correlation 

 or simultaneous evolution of organs. It proves, in effect, that 

 there exists, to a certain degree, a relative independence in the 

 evolution of organs; in other terms, a system of organs in this type 

 will be manifestly either behind or in advance of the stage of 

 evolution which it will have acquired in the majority of the group 

 to which the type in question belongs; or indeed the organ in 

 question will present characteristics entirely isolated. 



A simple remark will enable us in many cases to refer these 

 phenomena to principles already known and demonstrated. 

 Frequently aberrant forms constitute terms of transition between 

 two groups well marked and defined by the sum of their charac- 

 ters. This is the case in regard to the Prosobranchia monoto- 

 cardia and diotocardia, Which are delimited by important differ- 

 ences in the nervous system, the gills, the kidney, the heart, the 

 pallial sensory organs, etc. There are at least five or six forms 

 which are intermediate between the two groups in one or more 

 of these organs ; but, in these transition forms, one at least of the 

 organs which does not possess these characteristics of transition 

 is aberrant in regard to both groups. 



Another example may be drawn from types known only in a 

 fossil state. In the living world there is no term of transition 

 between the Arachnids and the Crustaceans. In the palaeozoic 

 epoch lived the Gigantostraca, of which the Limuli are at present 

 the last and much modified representatives. These animals 

 are in many characters intermediate between the Crustaceans 

 and the Scorpionidae, but at the same time they differ from each 

 of those. Between the Gasteropoda and the Acephala there is no 

 type of transition known, either extinct or living ; the only type 

 which presents indifferent characteristics is Dentalium^ which is, 

 moreover, very ancient, and is so aberrant that an especial class 

 has been erected for it. 



We will also mention the three types of the family, Gnetacea, 

 intermediate between the Gymnosperms and the Angiosperms. 



