344: 0. E\ Beecher — Origin and Significance of Spines. 



mal shows the second, third, fourth, and fifth digits of func- 

 tional 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 prehension than 

 locomotion, and in Iguanodon, the suppression of the pollex, 

 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. (D x .) 



The most obvious and direct relationship between an unfa- 

 vorable 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 present 

 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 ex- 

 clusively constituted the desert flora, but that a considerable 

 variety of types were 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 respects, may be consid- 

 ered as favorable, and also a type of plant life presenting evi- 

 dences of a deficiency of growth force. 



The great groups of plants commonly known as brambles 

 and climbing plants appear to meet most of the requirements. 

 They abound in regions where the greatest luxuriance of vege- 

 tation is found, and are therefore chiefly characteristic of the 

 tropics. Kerner 38 estimates that there are two thousand species 

 of the true climbing plants in the torrid zone, and about two 

 hundred in temperate regions. Tropical America has the 

 largest number of species, the flora of Brazil and the Antilles 

 being especially rich. In the sombre depths of the tropi- 

 cal forest, the climbing plants, or u lianes," are not so abund- 

 ant as in the open glades and along the edge of the forest, 

 where the amount of light is greater and the conditions of 



