On the other hand, Darwin himself and his followers have made almost endless experiments and observations on plants and animals, establishing facts as to structure and the relation of special kinds of living things to their surroundings which can only be explained on the supposition that Darwin’s theory is true in detail; that is to say, not merely that the kinds of animals and plants have arisen from previous kinds by natural descent—that supposition is much older than either Darwin or Lamarck—but that the method by which the transformation has been brought about is (_a_) the occurrence in every generation of every animal and plant of minute variations in every, or nearly every, part, and (_b_) the continual selection in the severe struggle for existence of those individuals to grow to maturity and reproduce, which happen to present favourable variations, which variations are accordingly transmitted to the next generation, and may be intensified, so far as intensification is of value, in each succeeding generation. A book full of observations and reflections about the structure, habits, and mode of occurrence and geography of a great number of plants and animals is Darwin’s _Journal of Researches_, published in 1845, and now republished as _A Naturalist’s Voyage_. In order to know very minutely the differences and resemblances between all the kinds or species of one group of living things Darwin studied for eight years the “cirrhipedes,” the name given to the sea-acorns and ships’ barnacles which occur in all parts of the world, some living on rocks, some on the backs of turtles, others on whales, on the feet of birds, on bits of floating wood or of pumice-stone, and some on one another! They are all hermaphrodites, but Darwin found in several a most singular thing, namely, the existence of minute males, complemental to and parasitic on the hermaphrodites. His discovery was doubted and denied, but he had the pleasure of seeing it at last fully confirmed thirty years after his book on cirrhipedes was published.