Of the most serious difficulties in the way of a theory of descent by evolution, the _first_ concerns _the nature of the evidence_, inasmuch as all change coming under observation does not indicate progression or improvement in the organism. The importance of this may be best indicated by quoting Mr. Darwin's explanatory words as to alterations in the fifth edition of his book on species. He says, "In the earlier editions of my _Origin of Species_, I probably attributed too much to the action of natural selection or survival of the fittest. I have altered the fifth edition of the _Origin_ so as to confine my remarks to adaptive changes of structure. I had not formerly sufficiently considered the existence of many structures, which appear to be, as far as we can judge, neither beneficial nor injurious, and this I believe to be one of the greatest oversights as yet detected in my work." When it is certain that deviation from the normal structure may take place which is a disadvantage to the individual, and that this may descend to offspring; when it is also shown that deviation may occur which appears to serve no end, that is, contributes to no phase of functional activity; when besides advantages gained are lost, and the race returns to its original type of structure; and when farther there are examples of degeneration, as in parasitic races,--such facts interpose special difficulties in the way of an all-embracing theory of progress by natural selection. Besides, as deviations occur of an unfavorable kind among domesticated animals under the care of man, it becomes obvious that progress may be readily lost even in most favorable circumstances.