as ether, alcohol, tobacco, lecithin, etc., (d) temperature, (e) oxygen, (f) presence or absence of waste products, (g) conditions of normal or abnormal stimulation and irritability. These extrinsic factors which influence growth have been studied by many investigators, but owing to lack of time I shall pass over all of them except the last named. In the case of the dwarf Crepidulæ which are found in the small shells with the small hermit crabs there is practically no evidence that any of the other factors except the last named, are involved in this dwarfing. These animals live in open shells on sandy sea beaches along with the giant forms; so far as I can determine, the food supply is superabundant, while the conditions of temperature, aeration and freedom from waste products are-identically the same for dwarfs and giants. The only difference which I have been able to detect is the size of the shells to which the animals are attached; those which are attached to the small shells of Nassa or Litorina live and die as dwarfs, reaching only about one thirteenth the volume of those which are attached to the larger shells of Natica; however, if they are removed from the smaller shells and placed on the larger ones they may grow to typical size. The dwarfs, however, are continually hampered by their limited quarters; they are unable fully to expand the foot or the mantle, and they are more frequently irritated by the movements of the hermit crabs than are those in the larger shells. Under these circumstances they probably take less food than those in larger quarters, and although they become perfectly differentiated and sexually mature they are dwarfed in size. Similarly I have found that Paramecium confined in capillary tubes never grows nor divides, though it may live indefinitely, and although precautions may be taken to change the medium frequently and thus to remove waste products and to supply abundant food and oxygen. In such tubes Paramecium is continually irritated and presumably takes less food than when in unconfined spaces.
II. Body Size, Cell Size and Cell Number
Is the size of an organism due to the size of its constitutent parts, or to the number of those parts, or to both of these causes combined? Evidently different organisms differ in this regard. In many plants and lower animals the number of constituent parts is directly correlated with the body size; branches and leaves, segments and organs may increase in number indefinitely with the growth of the organism. In tapeworms and many annelids the number of segments, with their characteristic organs, increases throughout life; but in more highly differentiated forms the number of body segments and organs is constant, and does not increase in number after embryonic stages. In spite of the information occasionally conveyed by examination papers, the
