necessary to keep them at a uniform temperature, if we wished to study their rate of development, and that is, for very practical reasons, extremely difficult and unsatisfactory. Far better it has seemed for our study of growth to turn to those animals which regulate their own temperature. This, accordingly, I have done, and the animal chosen for these studies was the guinea-pig, a creature which offers for such investigations certain definite advantages. It is easily kept; it is apt to remain, with proper care, in good health. Its food is obtainable at
all seasons of the year, in great abundance, and at small expense. The animals themselves being of moderate size do not, of course, require such extraordinary amounts of food as the large animals, should we experiment with them. Accordingly with guinea-pigs I began making, years ago, a long series of records, taking from day to day, later from week to week, and then, as the animals grew older, month by month, the weight of recorded individuals. There was thus obtained a body of statistics which rendered it possible to form some idea of the rapidity of growth of this species of mammal.
Now in regard to the rapidity of growth, it is necessary that we form clearer notions than perhaps you started out with when you came into the hall this evening. I will ask for the next of our pictures on the screen, where we shall see illustrated to us older methods of
