It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.

Morphologists have been slow in realizing that the basic principles of Darwin's theory of evolution by natural selection meant that understanding the evolution of anatomical features must be based on an analysis of how these structures interact with demands of the external environment on the organisms.