The theory of descent with modifications produced through the action of natural causes has been accepted by the best scientific minds of the age as expressing nature's method in the elaboration of organic forms. In no other way, perhaps, has this doctrine been of such signal service to science in general as through its influence upon the methods of studying and interpreting natural phenomena. Especially in the biologic sciences has this principle of development, of progressive modification, furnished the key for the solution of numerous problems. In the light of this principle, differences of kind are seen to arise through the accumulation of differences of degree, and, further, that these organic modifications arise through adaptation to the condition of life. The life of an organism is, in fact, a series of adjustments to varying conditions and forces which affect it.
Hitherto, our observations and generalizations have related chiefly to the