34 A TREATISE ON METAMORPHISM. 



it is advantageous to the animal under the conditions in which it lives. 

 Since adaptation is an assumed law, in those cases where there seems to be 

 lack of adaptation, as where some peculiar structure is present which 

 apparently is not of advantage to an animal or plant, it is believed that the 

 facts are not fully known or that the structure was once useful and is a 

 survival. However, the very idea of survival shows that to a certain degree 

 the development of plants and animals lags behind their changing environ- 

 ment. Upon a priori grounds it would be certain that this is the case ; and 

 the existence of rudimentary organs, such as the muscles for moving the 

 human ear, which at one time may have had a use, is positive evidence of 

 the lag of organic species during their adaptation to changing environment. 



Likewise it is believed that minerals constantly tend to change to 

 forms that are relatively stable under existent conditions. This, however, 

 is accomplished by granulation or recrystallization or some analogous 

 process, and is adaptation only in the sense that the old particles break 

 up into smaller particles or develop into new mineral particles which 

 conform to the existent conditions. Some minerals are stable under a 

 considerable variety of conditions, and therefore are less sensitive to 

 change than are others. For instance, quartz develops directly from an 

 igneous rock, and it also forms as a deposit from water. It persists under 

 both quiescent and dynamic conditions. Other minerals require rather 

 definite conditions for their existence. Such are leucite and olivine, which 

 abundantly form as original minerals in igneous rocks of certain composi- 

 tion, but which readily change under new conditions to other minerals. 

 However, no mineral persists without reference to its environment, and so 

 it may be said that there is a tendency in all mineral substances to form 

 minerals adjusted to the conditions under which they exist. Rocks are 

 composed of aggregates of different minerals. Therefore rocks, like 

 minerals, have a tendency toward adjustment to their environment. 



Even if the chemical composition of a small mass, say a cubic milli- 

 meter, remains exactly the same, the mineral constituents of the mass 

 may greatly change. At the end of the change the original minerals may 

 not be in the same proportions as before and minerals which did not 

 originally exist in the rock may have formed. But the adjustment of rocks 

 is not confined to redistribution of the elements present in a small space. 

 There may be a change in the average chemical composition of rocks. 



