26 STUDIES IN EVOLUTION 



Of equal significance is the fact that the Echinodermata 

 together with the plants represent the most primitive tj-pe of 

 structure, one in which there is a more or less circular 

 arrangement of the parts or organs. The Echinodermata 

 are the highest development in this line of growth among 

 animals. They culminated in past geological ages, and from 

 them no direct line of descent can be traced (Bailey^ and 

 Cope"). 



The conclusion from the study of the phylogenies of 

 spinose forms is parallel to the one drawn from the ontogenies ; 

 namely, that the ancestors of spinose as well as non-spinose 

 organisms were simple and inornate. 



Categories of Oeigin 



As previously shown, spines are formed either by growth 

 or by suppression, and therefore the processes determining 

 their production are either constructive through concrescence 

 or destructive through decrescence. Each of these is in turn 

 determined by forces from without the organism (extrinsic) 

 or by forces from within (intrinsic). In this connection it 

 is of no especial moment whether or not the intrinsic forces 

 are primary or are an immediate or subsequent reflex from 

 the extrinsic. The main thing is the direction of the dom- 

 inant force, whether centripetal or centrifugal. If in some 

 cases it can be shown that spine development has been 

 accomplished by intrinsic forces in the organism, then this 

 development may be brought about independently of the 

 environment and possibly at variance with it. Also, if in 

 other cases the extrinsic forces or the influences of the 

 environment have caused spine growth, it may in some 

 instances illustrate the formation and transmission of an 

 acquired character, or at least the operation of organic 

 selection. 



The point has now been reached where it is impracticable 

 to make a rigid classification of the direct factors or an 

 exact determination of primary and secondary causes. It 



