Environment and Adaptation 221 



ages of grazing animals, but there is no question 

 that these peculiarities have enabled the plants to 

 survive and flourish, in spite of constant cropping. 



Protection Against Animals. Somewhat differ- 

 ent are the special protective devices found in many 

 plants which enable them to repel the attacks of ani- 

 mals. These are especially marked in plants of arid 

 regions where the life conditions are precarious. 

 The development of defensive armor, like the terri- 

 ble spines of the cacti, and the dagger leaves of the 

 century plant and yuccas, as well as the rank secre- 

 tions of the sage-brush and creosote-bush, are very 

 efficient weapons against the attacks of hungry ani- 

 mals ; and although they may be only physiological 

 responses to the arid environment, they are never- 

 theless exceedingly useful to the plant as protective 

 measures, and must have been of immense impor- 

 tance in preserving plants in the very unfavorable 

 conditions surrounding them. 



Myrmecophily. The insectivorous habit of cer- 

 tain plants has already been referred to, but another 

 extraordinary association with insects may be briefly 

 cited, as it is one of the most remarkably reciprocal 

 adaptations known to the naturalist. This is the 

 habit discovered in certain ants of associating them- 

 selves with plants in a sort of symbiotic relation, 

 which has been termed " myrmecophily." In a 

 number of trees, notably the genus Cecropia in 

 tropical America, and certain species of Acacia, 

 the trees harbor colonies of ants which inhabit 



