To the naturalist many instances will readily occur of remarkable changes of form during the evolution of an animal from the ovum to the mature stage. Steenstrupp, the Danish naturalist, in 1845 summarized the process of development in the Medusae, Entozoa, and others of the lower animals, under the title of " Alternation of Generations", which he described as " the remarkable and till now inexplicable natural phenomenon of an animal producing an offspring, which at no time resembles its parent; but which, on the other hand, itself brings forth a progeny which returns in its form and nature to the parent animal; so that the maternal animal does not meet with its resemblance in its own brood, but in its descendants of the second, third, or fourth generation." This remarkable form of evolution is exhibited in the reproductive process of the parasite the liver fluke (Distoma hepaticum) in the most striking manner. The parent fluke provides the ovum, and there its responsibility seems to cease. Hatching takes place in any moist spot or stagnant pool. The product, however, is not a young fluke, but a Jong, thin embryo, having no resemblance to the parent. Soon, however, this undergoes a change into a cyst, or sort of bag, in the interior of which are developed more advanced organisms known as redia, and in them, again, still more advanced tailed cercaria appear, which are nearest to the form of the fluke, and only await entrance into the body of a warm-blooded animal to acquire their perfect form and thus prove their descent from their original parent. (See p. 260 of this volume.)

Equally remarkable transformations occur during the development of the embryo in the higher animals, but these go on within the organism of the parent, or otherwise while the young animal is enclosed in the shell of the egg. It is, however, possible to imagine that the changes which occur in the embryo, which is hidden from sight, as in the egg of the bird or the uterus of the mammalian, might be displayed to view, as it is in some of the Entozoa and other animals lower in the scale of life. What a wonderful series of phenomena would be exhibited! Instead of sitting on her eggs for weeks, the common hen would find her brood at the expiration of a few days hatched but palpably unfinished, very unlike the chickens to which she had been accustomed. The young living beings would present some of the characteristics of the mammal, but they would also, in certain parts of their organism, show structures connecting them with reptiles, and, in the arrangement of the blood-vessels, they would run the risk of being classed among fishes.

Day by day almost imperceptible changes would be distinguished by the critical observer, and gradually the unnamed living thing would assert its claim to be accepted as a bird, and finally the expert would decide, without the least chance of making a blunder, to which particular species, genus, and variety of birds the mysterious creature belonged. In like manner it might be supposed, for the sake of illustration, that the mare, instead of producing a foal at the end of several months, would give birth at an earlier period to an unfinished organism in which some of the characteristics of the lower Vertebrata would be recognized, those of the reptile or the fish, for example. Gradually advancing day by day, the young organism would exhibit in turn, in the circulatory system especially, some of the features belonging to the bird, and passing through the changes which mark certain phases in the organization of the lower mammals, it would arrive at the stage of perfect development and assume the form of the parent.

In reply to the very easy and obvious criticism that the above description is absurd, and that the processes referred to could not possibly take place, it may be observed that nothing has been advanced of a hypothetical character. All the changes or metamorphoses referred to do constantly occur, and the only liberty which has been taken has been that of supposing them to be visible. Facts which are invisible to the ordinary observer, but perfectly distinct to the skilled microscopist, have been assumed to occur in such a position that they might be recognized by the unaided eye. The facts are not the less real because they do not present themselves in a palpable form. In the study of embryology all these changes are recognized, but they are visible only by the aid of the microscope. If they formed part of the ordinary observation of the breeder of stock, as they do of the investigations of the scientist, the mysterious doctrine of evolution would lose its glamour, and become one phase of mere commonplace experience.

Replying to some of the objections which have been urged against the doctrine of evolution, the late Herbert Spencer deals with two forms of criticism which have often been advanced, one relating to the obvious and admitted fact that the process of evolving a new species has never been seen, and the other to the difficulty which is based on the ground of the extent of time which would necessarily be required for the development of highly-organized living creatures out of a mass of jelly-like protoplasm.

On the first point he quotes from the late Lord Salisbury's address to the British Association, in which the speaker says that no man or succession of men have ever observed the whole process in any single case, and certainly no man has recorded the observation. In reply, Herbert Spencer quotes from an essay which was published many years ago in pre-Darwinian days, in which the author remarks: " In a debate upon the development hypothesis lately narrated to me by a friend, one of the disputants was described as arguing that as, in all our experience, we know of no such phenomenon as transmutation of species, it is unphilosophical to assume that transmutation of species ever takes place. Had I been present, I think that, passing over his assertion, which is open to criticism, I should have replied that, as in all our experience we had never known a species created, it was by his own showing unphilosophical to assume that any species ever had been created."