86 STUDIES IN EVOLUTION 



The first is represented by a slender tarsal bone alone, 

 while the fifth is completely suppressed. The manus, or 

 fore foot, of this animal shows the second, third, fourth, and 

 fifth digits of functional importance as digits, while the first 

 is shortened and atrophied to the condition of a stout spur, 

 standing out at right angles to the axis of the leg, as shown 

 in figure 70. The fore legs of Iguanodon and others of the 

 same order were short, and apparently used more for prehen- 

 sion than locomotion, and in Iguanodon the suppression of the 

 poUex, or thumb, into a spur doubtless provided the animal 

 with a powerful weapon. Here is seen the suppression of a 

 digit by loss of normal function, resulting in a protective 

 structure of considerable value. 



XI. Intrinsic suppression of structures and functions. (Di.) 



The most obvious and direct relationship between an un- 

 favorable environment and the suppression of structures to 

 form spines was afforded by desert plants. In illustration of 

 the intrinsic suppression of structures by deficiency of growth 

 force, the vegetable kingdom again seems to offer the clearest 

 evidences of a like relation between cause and effect. Instead, 

 however, of taking an unfavorable environment, in the pres- 

 ent instance a favorable environment must be assumed, and 

 then a type which expresses in various ways its deficiency of 

 growth force must be sought. 



In the desert plants it was found that no single family 

 exclusively constituted the desert flora, but that a consider- 

 able variety of types was present, and that some of these 

 belonged to perfectly normal families commonly living under 

 ordinary favorable conditions. Moreover, it was evident that 

 there were certain types of form and habits of growth which 

 were especially characteristic of plants living in desert or 

 similar unfavorable regions. Therefore, to illustrate clearly 

 intrinsic restraint or suppression of structures it will be 

 necessary to take an environment which, in most re- 

 spects, may be considered as favorable, and also a type of 



