Another possible cause of degeneration appears to be the indirect one of minute size. It cannot be doubted that natural selection has frequently acted on a race of animals so as to reduce the size of the individuals. The smallness of size has been favourable to their survival in the struggle for existence, and in some cases they have been reduced to even microscopic proportions. But this reduction of size has, when carried to an extreme, resulted in the loss or suppression of some of the most important organs of the body. The needs of a very minute creature are limited as compared with those of a large one, and thus we may find heart and blood-vessels, gills and kidneys, besides legs and muscles, lost by the diminutive degenerate descendants of a larger race. That this is a possible course of change all will, I think, admit. It is actually exemplified in Appendicularia—the only adult representative of the Ascidian tadpole—still tadpole-like in form and structure, but curiously degenerate and simplified in its internal organs. This kind of degeneration is also exemplified in the Rotifers, or wheel animalcules, in the minute Crustacean water-fleas (Ostracoda), and in the Moss-polyps, or Polyzoa. Roughly then we may sum up the immediate antecedents of degenerative evolution as, 1, Parasitism; 2, Fixity or immobility; 3, Vegetative nutrition; 4, Excessive reduction of size. This is not a logical enumeration, for each of these causes involves, or may be inseparably connected with, one or more of the others. It will serve for the present as well as a more exhaustive analysis. (See Note C.)