Until recently we might have answered, 'external conditions'; it is they that load the dice one-sidedly, and condition that the same straight path of phylogenesis is adhered to, and exactly the same direction of variations is preferred and maintained. It has to be asked, however, whether this answer, which is certainly not absolutely incorrect, is sufficient by itself, whether the dice are not falsified and one-sidedly loaded in another sense, so that they always throw a preponderating number of the useful variations. We shall attempt very soon to solve this problem, but in the meantime I must refer to another argument in favour of assuming the Lamarckian principle, perhaps the most important and it may be thought the most difficult of all to refute, the so-called co-adaptation of the parts of an organism, that is, the fitting together of many individual organs for a common purposeful functioning. LECTURE XXIV OBJECTIONS TO THE THESIS THAT FUNCTIONAL MODIFICATIONS ARE NOT TRANSMITTED Giant stag as an example of co-adaptation or 'harmonious adaptation'--This occurs even in passively functioning parts--Skeleton of Arthropods--Stridulating organ of ants and crickets--Limbs of the mole-cricket--Wing-venation--Colorations which form mimetic pictures--Harmonious adaptations in worker-bees and ants--Degeneration of their wings and ovaries--The quality of food acts as a liberating stimulus--Vom Rath's case of drones fed with royal food--Transition-forms between females and workers--Wasmann's explanation of these--The Amazon ants--Two kinds of workers--Appendix: Zehnder on the case of ants--On the skeleton of Arthropods--Hering's interpretation of Ehrlich's Ricin experiments--Hering's position in regard to the transmission of functional modifications.