ORIGIN AND SIGNIFICANCE OF SPINES 95 



over-production or extravagant differentiation, followed by a 

 decline of spinous growth, and ending in extreme senility 

 with their total absence. 



There are abundant reasons for believing that the radicles 

 of groups are undifferentiated and inornate, and whenever a 

 class has had a long existence it has been by the continuance 

 of such radical types or by the development of secondary or 

 tertiary radicles, which, though differing in internal charac- 

 ters, still retain a primitive simplicity in superficial features. 

 The early stages of ontogeny of any form should agree with 

 the radical stock, and, as already noted, these stages are 

 simple. Hyatt 34 says on this point: "the evidence is very 

 strong that there is a limit to the progressive complications 

 which may take place in any type, beyond which it can only 

 proceed by reversing the process and retrograding. At the 

 same time, however, the evidence is equally strong that there 

 are such things as types which remain comparatively simple, 

 or do not progress to the same degree as others of their own 

 group. Among Nautiloidea an4 Ammonoidea these are the 

 radicle or generator types. No case has yet been found of 

 a highly complicated, specialized type, with a long line of 

 descendants traceable to it as the radicle, except the progres- 

 sive ; and all our examples of radicles are taken from lower, 

 simpler forms; and these radicle types are longer-lived, 

 more persistent, and less changeable in time than their 

 descendants." 



A few examples will now be taken from the life histories 

 of large groups. In the Brachiopoda the order Protremata, 

 containing most of the spinose forms, has 4 genera and 22 

 species in the Cambrian of America, 20 genera and 173 

 species in the Ordovician, and 30 genera in the Silurian. 

 "Then began a steady decline, with extinction in the Car- 

 boniferous of North America. In the Triassic of Europe 

 this order is sparingly represented by small species, and is 

 there essentially restricted to the family Thecidiidse, which 

 continues to have living representatives in the Mediterranean 



