These two books were written simultaneously by two professors of the University of Chicago, published simultaneously and evidently intended to be mutually complementary and supplementary, as they were written each with the knowledge of the other's manuscript.

Child's book is an extensive, further elaboration and, for the time being at least, a final statement of the work he has been engaged upon now some years. It is a general biological discussion based upon experimental laboratory material of his theory of dynamic gradients as the basic