Initiative in Muscles. Initiative in the evolution of muscles clearly occurs somewhere in the stem, and behind the formed expression of an altered habit is the integrating action of the nervous system. This will be by some looked at askance as a _deus ex machinâ_ and reckoned as part of the argument from ignorance in a way which recalls Weismann’s scorn of Lamarckian factors in germinal selection. I submit that what he and Osborn call “the unknown factor” of use and habit, arising in response to new stimuli meets as no other proposed suggestion does the formation of new muscles. Given a certain fundamental architecture of skeleton and musculature, such as of primitive vertebrates, one can, without doing violence to any known facts, place the formation of new organs of movement in the following order:-- 1. Neural changes and habits. 2. Muscular modifications. 3. Consequent modifications of bone. It carries the question no further to say that these are correlated, however loose may be the meaning of that word that is understood. If the prerogatives of Selection within the germ, of segregation of unit-characters and dominance, and of mutations are not unlimited in the construction of organisms, there still remains a sphere of action for the initiating power of the nervous system. Bones grow and change their form in response to increased or altered muscular action on them, and it is necessary to look back a stage further in the story to the neural changes however produced. There have been abundant opportunities in the long history of mammalian evolution for primitive forms to take a new course of life, and they have done so on an extensive scale. The impulses that have led them may have been started by some “needs” such as Lamarck taught, some change in their surroundings involving new stimuli, or “insults,” as Haeckel called them, but the first of the structural stages must have been in the cerebral cortex. Cross-Roads in Evolution.