cell has made of itself by sub-dividing and differentiating.  In the process, the cell mass often goes through stages which stand out as individualities in themselves, that appear on the surface absolutely unrelated.  So the caterpillar and the butterfly, to the naive child, seem as far apart as worm and bird.  In the case of the frog, the tadpole as a first sketch seems completely an impossible and wild absurdity.  Yet we know that there is an orderly progression of events, a propagation of cells, a forward going arrangement of chemical reactions, that results in expansion and intricate complication of the organism.  Just what the forces at work in this most mysterious of all natural processes are, has been an intellectual mystery that the best minds of the race have attempted to get rid of with words like pangenesis (Darwin).  Words of Black (Mediterranean or Greek and Latin) origin, as Allen Upward has named them, always cover a multitude of ignorances.  The glands of internal secretion, here, as in so many other dark places, provide the open sesame to certain long closed doors of biology.  They offer themselves to us as the first definitely tangible agents which are known to keep the process of growth going, and undoubtedly initiate the marvelous unfolding of tissues and functions, organs and faculties summed up as development or differentiation.

Thus by the direct feeding of thyroid at particular points in the differentiating history most curious effects have been elicited.  If the gland is made part of the nutriment, the bathing environment, of the tadpole, a hastening of its metamorphosis is attained.  The tadpole lives not out its day as a tadpole, but precociously turns into a frog.  But such a frog!  It is a miniature frog, a dwarf frog, a frog seen by looking through the wrong end of the telescope, a frog not magnified, but micrified.  Frogs have been so created the size of flies.  There has occurred a splitting of the two reactions which ordinarily go hand in hand:  the reaction of growth which is just brute increase of total mass or weight and volume, and the reaction of differentiation which is the finer process.  The picture is a frog, but a frog the size of a tadpole, a frog which has missed its childhood, adolescence and youth, skipping over these transition stages into the adult age, as a pigmy.

It is all as if a baby were suddenly to grow a beard and moustache, evolve and shed teeth, and acquire the manner of an earnest citizen, and yet retain the height and weight of a baby.  That the spectacle of such a superbaby is not quite the most fantastic of all improbabilities is shown by the condition of progeria, first recorded by the Briton, Hastings Guilford.  A queer spectacle in which a child incontinently grows old without having lived—­in the course of a few weeks or months.  You look upon him and see senility on a small scale, but with all its peculiarities:  wrinkled skin, apathy, gray hair and all the rest