Perhaps no principle has ever been announced so fertile in results as that which Mr. Darwin so earnestly impresses upon us, and which is indeed a necessary deduction from the theory of Natural Selection, namely — that none of the definite facts of organic nature, no special organ, no characteristic form or marking, no peculiarities of instinct or of habit, no relations between species or between groups of species — can exist, but which must now be or once have been useful to the individuals or the races which possess them. This great principle gives us a clue which we can follow out in the study of many recondite ph?nomena, and leads us to seek a meaning and a purpose of some definite character in minuti? which we should be otherwise almost sure to pass over as insignificant or unimportant.