EOCKS ADAPTED TO ENVIRONMENT. 33 



set of conditions, minerals tend to form which remain permanent under 

 those conditions. This tendency is more potent with minerals crystallizing 

 from magmas than with minerals which constitute the sedimentary rocks 

 or with the secondary minerals which form by metamorphism. The reason 

 for this is that adjustment to existing conditions is so much more readily 

 accomplished in fluids than in solids; but the tendency to form minerals 

 which are permanent under the existing conditions controls in the solid 

 rocks, although there is a great amount of lag in the process of modifica- 

 tion. If adjustment be reached in a given case and if the conditions 

 remain the same, the minerals formed do not again alter, but may remain 

 the same through eons. This is illustrated by the meteorites, the minerals 

 of which may persist without change during the evolutions of stellar sys- 

 tems. However, when an important change of conditions occurs, as when 

 a meteor gives up its separate existence in the interstellar spaces and joins 

 a planet, as the earth, readjustment begins at once. 



Although the changes of conditions upon the earth are not so great as 

 the change when a meteor falls to the earth, the range of conditions upon 

 the earth is large and varied. The conditions may be those of ordinary 

 pressure and temperature at or near the surface of the earth, or they may 

 be those of very high pressure and temperature, such as exist well below 

 the surface of the earth. A rock mass may alternately be subject to each 

 of these sets of conditions and to various intermediate conditions. Changes 

 of physical conditions result from surficial transfer of material by epigene 

 agents — bringing rocks to the surface here, burying them there — from 

 igneous intrusions, from orogenic movement, and from other causes. The 

 changes upon the earth are therefore profound, although usually slow. 



During the changes the rocks are always modified in the direction of 

 adjustment to the new conditions. Such modification of rocks has led to 

 the idea of adaptation to their environment. As conditions change, species 

 of plants and animals are so rapidly modified that at first sight adaptation 

 seems almost perfect. Indeed, so sensitive are plants and animals to their 

 environment that since the theory of evolution gained ascendancy the fact 

 of approximate adaptation is taken for granted. The variety and complexity 

 of the structures, colors, etc., of life forms resulting- from adaptation to 

 environment is a constant source of wonder. Almost daily some remarkable 

 structure or form is described, and its existence explained by showing how 

 mon xlvii — 04 3 



