450 Proceedings of the Boyal Physical Society. 



(1.) The first division includes what maybe called molar or mechanical 

 influences, such as the lateral pressures of currents in air or water, the vertical 

 pressure of gravity or of superincumbent masses, and also the amount of space 

 at the disposal of the orcranism. 



(2. ) The second division or sheath of influences comprises the influence of 

 food, both as regards quantity and quality, the amount of oxygen available, 

 the quantity of water, the chemical composition of the medium, including 

 soil, water, air, interior of other organisms, and so on. 



(3.) The third series are the higher forms of energy, — heat, light, electricity, 

 and magnetism. 



(4.) The fourth division includes influences due to other organisms, whether 

 those be simply neighbours, casual associates, true commensals, parasites, or, 

 most intimately of all, symbions. Under this head the influence of man in 

 his work of domestication might also be noted. 



5. Review of Concrete Pesearches. — Under the headings just 

 indicated, some illustrative researches must now be noted. 

 Some of those cited are already familiar and classic, others 

 perhaps less so. The text must be read along with the 

 classified bibliography, which is simply intended as an 

 appendix to Semper's well-known work, " The Natural Con- 

 ditions of Existence as they affect Animal Life" (1880), to 

 which I cannot too strongly express my obvious indebtedness. 

 The majority of the researches here referred to are of a later 

 date than those given by Semper ; and it only remains to be 

 added that, in several cases where the literature is legion, 

 {e.g., on the influence of light on vegetation, and on the 

 general problem), only sample researches are noted. From 

 most of the papers quoted, indeed, the list can be extended. 



I. Molar or Mechanical Influences. 



(Bibl., 1-34.) 



(a.) Amount of >S^«ce.— Semper (28, 29) reared dwarf 

 broods of Asellus and Lymnceus in confined space. Yung 

 (34) has shown that the fewer tadpoles within a given area 

 the quicker the development; and Balaschewa (2) has 

 observed the relations between molluscs and the size of the 

 water basin in which they live. 



(6.) Lateral Pressures. — The cutting up of aquatic leaves 

 (5, 13, 17, 33, 215), the moulding of coral forms (30), the 

 chiselling of shells (30), the general shape of many aquatic 

 swimmers (1), have been explained in association with the 



