24 THE MAMMALIA. 



it appear extremely probable that these eyes of 

 different species and individuals originated inde- 

 pendently of one another. By connecting this 

 development with the general characteristics of the 

 cells of the upper skin and of the protoplasm, he 

 shows that this repeated formation of eyes must be 

 regarded as arising from a simple foundation. This 

 is one form of convergence, repetition in the indivi- 

 dual. The other is contained in the fact that the 

 eye of the onchidium shows an advance towards 

 the eye of the vertebrate, inasmuch as its structure 

 and the retina differ essentially from the eye of the 

 other molluscs. 



A few years ago a great fuss was made about a 

 case of convergence which scarcely deserved this 

 name, and when inquired into dwindled down to 

 certain superficial resemblances, such as have been 

 observed times without number since we have had 

 descriptions of natural objects. I refer to the re- 

 semblance of certain primeval reptiles to mammals, 

 the so-called Theriodonta. The case was this. 1 In 

 the Trias formation of the southern extremity of 

 Africa there were found, in addition to the colossal 

 plant-eating reptiles of the group Dinosauria, a 

 series of other animals, hitherto unknown, which, 



1 Owen, Fossil Rcptilia of South Africa. London, 1876. 



