_Dysteleology, or the theory of purposelessness_ [is] the name I have given to the science of rudimentary organs, of suppressed and degenerated, aimless and inactive, parts of the body; one of the most important and most interesting branches of comparative anatomy, which, when rightly estimated, is alone sufficient to refute the fundamental error of the teleological and dualistic conception of Nature, and to serve as the foundation of the mechanical and monistic conception of the universe. It will be sufficient to quote Professor Huxley's remarks upon this passage, taken from the very laudatory review he wrote of the work in which it occurs.[225] Professor Haeckel has invented a new and convenient name, "Dysteleology," for the study of the "purposelessnesses" which are observable in living organisms--such as the multitudinous cases of rudimentary and apparently useless structures. I confess, however, that it has often appeared to me that the facts of Dysteleology cut two ways. If we are to assume, as evolutionists in general do, that useless organs atrophy, such cases as the existence of lateral rudiments of toes in the foot of a horse place us in a dilemma. For, either these rudiments are of no use to the animal, in which case, considering that the horse has existed in its present form since the Pliocene epoch, they surely ought to have disappeared; or they are of some use to the animal, in which case they are of no use as arguments against Teleology. A similar, but stronger argument may be based upon the existence of teats, and even functional mammary glands in male mammals.... There can be little doubt that the mammary gland was as apparently useless in the remotest male mammalian ancestor of man as in living men, and yet it has not disappeared. Is it then still profitable to the male organism to retain it? Possibly; but in that case its dysteleological value is gone. In later editions Professor Huxley further observed: "The recent discovery of the important part played by the Thyroid gland should be a warning to all speculators about useless organs."[226] It seems, therefore, the wiser part to refrain from basing any vital conclusions upon these organs until we can assure ourselves that our knowledge warrants our so doing. As the same Professor Huxley intimated, it might be well for palæontologists, and doubtless for biologists likewise,[227] "To learn a little more carefully that scientific '_ars artium_,' the art of saying 'I don't know.'"