Injlitence of Environment upon the Organism. 451 



pressure of the surrounding medium ; and many passive 

 organisms from sponges to trees afford abundant illustration 

 of such shaping influence. 



(c) Vertical Pressure. — Alteration of pressure affects the 

 growth of plants (33) ; Eauber and Sachsse (22), experiment- 

 ing on embryos, showed that augmented pressure tended to 

 alter the shape, e.g., to increase the breadth, while lessened 

 pressure evoked dropsical forms ; Eegnard (23), Certes (9), 

 and otliers, have shown that very high pressure induces latent 

 life, etc. ; and Romanes (24) has also been working at the 

 effect of pressure on excitable tissues. Under this head the 

 influences of altitude (6, 7, 18), bathymetrical distribution, 

 and the like, ought to be included. Gravity itself is known 

 to be an important factor, influencing phenomena so diverse 

 as the position of material in the ovum before and during 

 segmentation, cell-division, growth of trees, and shapes of 

 mollusc shells (8, 10, 11, 11a, 14, 19, 20, 21, 31, 33). Some 

 other mechanical influences difficult to classify are referred 

 to in the bibliography. 



II. Chemical Influences. 



(Bibl., 35-llL) 



{ct) AmoitJit of Oxygen. — In a broad shallow vessel with 

 consequently good aeration, Yung's tadpoles (70) developed 

 more rapidly. Gratacap (47) observes that increased oxygen 

 makes insects at first very restless and active. An overplus 

 of oxygen evokes strong movements in a Parariimcium., and, 

 according to Eauber and Sachsse (57), quickens the embryonic 

 development of fishes and amphibians in the region of the 

 gill-slits. A superfluity of oxygen has been shown (51) to 

 afl'ect the formation of pigment in frogs. On the other hand, 

 Fiszer (41) observed that deficient oxygen quickens the 

 pulsations of the contractile vacuoles of Infusorians, just as 

 it would make a flame leap and quiver. 



(h) Amount of Moisture. — Drought is often followed in 

 some of the loweranimals by encystation, desiccation, and latent 

 life. A Pontederia has a spongy airy stem in water, very 

 different from that which it exhibits when on shore; and 

 other changes in plants have been associated with the amount 



