98 STUDIES IN EVOLUTION 



and motor defence, the motion growing more sluggish as the 

 defensive armor grows more efficient. But in the whole king- 

 dom motion persists as one of the defensive agencies. No 

 animal exists without some power of motion, by whose aid it 

 withdraws or otherwise escapes from danger." He also notes 

 that the plant kingdom, with the exception of the minute, 

 swimming forms, possesses no defensive motion, and that 

 mechanical defence alone exists. Under mechanical defence 

 are included thorns, spines, etc., together with chemical appli- 

 ances ; as in plants with poisonous or disagreeable juices. 

 These facts lead to the conclusion that, in proportion as 

 animals are spinose or armored, they exhibit a vegetative type 

 of structure, and have retrograded. 



It has been shown elsewhere in this article, that the great- 

 est development of spinose organisms occurs just after the 

 culmination of a group, and, as this period clearly represents 

 the beginning of the decline of the vitality of the group, the 

 spines are to be taken as the visible evidence of this decadence. 

 A similar observation has been made by Packard,^ who after 

 passing in review the geological development of the Trilobita, 

 Brachiopoda, and Ammonoidea, states that " these types, as is 

 well known, had their period of rise, culmination, and decline, 

 or extinction, and the more spiny, highly ornamented, abnor- 

 mal, bizarre forms appeared at or about the time when the 

 vitality of the type was apparently declining." 



Furthermore, it is now commonly agreed that all groups 

 have been most plastic near their point of origin, or, in other 

 words, that during their early history all the important or 

 major types of structure have been developed. Their subse- 

 quent history reveals the amount of minor differentiation and 

 specialization they have undergone. Apparently, most of the 

 early impulses of growth, whether from the environment or 

 from vital forces, resulted in physiological changes producing 

 fundamental variations in function and structure. The later 

 influences of environment and growth force are expressed in 

 peripheral differentiation, and show that the racial or earlier 

 characters had become fixed, and that the later or specific 



