3i6 DESIGN IN NATURE 



Elasticity can only come into play when the substance which exhibits it is put upon the stretch or is crushed 

 up, and this stretching out and crushing up in plant and animal movements is, in every instance, due to a vital 

 operation. Elasticity cannot act of itself, and when it is made to act, it acts by recoil and produces a jerky move- 

 ment ; but (and this is the curious thing) when jerky movements are witnessed in plants and ammals they are 

 caused not by elasticity, but by hfe ; for example, by the sudden contracting of living cells and hollow viscera, 

 with a view generally to the propulsion of fluids, and by the energetic contraction of muscles, with a view to pro- 

 ducing movements in the travelling organs or other parts of the body. The sudden movements referred to are 

 vital movements ; movements to a given end and for a purpose. Elasticity, when evoked by life, is always 

 under control, and it is a mistake to assign to it anything but the most subordinate rols in plant and animal 

 physics. One has only to watch the movements of protoplasm, of the white blood-corpuscles, of the amoeba, the 

 sarcous elements of muscles, &c., to be convinced of this. It is an error to suppose that vital movements are only 

 manifested by contractions, and by elasticity as a counter-acting force. The vital movements are numerous and 

 various : pushing or elongating movements, pulhng or shortening movements, contracting or closing movements, 

 expanding or opening movements, spiral movements, &c. The pushing, elongating, opening movements are centri- 

 fugal in their nature ; the shortening, contracting, closing movements being centripetal. The terms centrifugal 

 and centripetal are employed in relation to the central portions of the substance exhibiting the movements. The 

 movements in the hving mass, as a rule, spread from and return to a given point ; that point, for the most part, 

 corresponding with the position of rest. That hving protoplasm can advance and retire as apart from contraction 

 and its supposed counter-acting force of elasticity, is proved by the movements of chmbing, sensitive, and insectivorous 

 plants ; by the advance of the plasmodium of Badhamia vtricularis ; by the amoebic movements of the white blood- 

 corpuscles ; by the movements of the amoeba, and by the streaming of the pseudopodia of Gromia, and the 

 Foraminifera generally. The plasmodium of Badhamia vtricularis when feeding advances as a wedge-shaped mass, 

 no trace of contraction being anywhere observable. The white blood- corpuscles when forcing themselves through 

 the walls of the capillary blood-vessels, as I have satisfied myself by careful microscopic examination, invariably 

 throw out a knuckle or wedge of their substance, the thin end of which perforates the vessels. When the 

 corpuscles have in part forced a passage, they draw or pull the remainder of their substance through the capillary 

 walls, where they assume their original shape. Similarly, the amoeba invariably moves by projecting one or more 

 wedge-shaped portions of its substance — the substance streaming towards the apex of the moving wedge or 

 wedges. The amoeba extends its substance in the direction of travel, and, in a sense, pushes itself forward by 

 a centrifugal movement. There is no constriction in the advancing mass to indicate contraction of any kind, 

 and no trace of elasticity can be detected. The advancing mass deliberately — that is, voluntarily — thrusts itself 

 forward in an aggressive manner. The white blood- corpuscles and the amoeba exhibit both centrifugal and 

 centripetal movements. 



The movements of pseudopodia as seen in Gromia are also centrifugal and centripetal in character ; when Gromia 

 is searching for food, it extrudes its protoplasm or body substance voluntarily in continuous streams in more or less 

 straight hnes. When prey is caught the protoplasm is voluntarily and suddenly drawn towards and into the central 

 mass. The outward centrifugal movement is a vital, non-elastic, advancing, aggressive movement made in search 

 of food ; the inward, centripetal, retreating, non-aggressive movement (also vital) represents the remaining or com- 

 plementary half of the capturing act. The centrifugal and centripetal movements are co-ordinated, complemental, 

 and voluntary, and neither movement would be of any use by itself. The movements of the sarcode of Gromia 

 (and of the amoeba) are in all respects analogous to the movements of the white blood-corpuscles and the sarcous 

 elements of muscle, voluntary and involuntary. If a fibrilla of a hving, striated, voluntary muscle be examined 

 under the microscope it will be seen that the more or less square, cube-shaped sarcous elements forming it are 

 endowed with a double power, whereby they can elongate first in one direction, and then in another and opposite 

 direction. When all the sarcous elements of the fibrilla elongate in the direction of the length of the fibriUa, the 

 fibrilla is said to relax, in which case it increases in length ; when all the sarcous elements elongate transversely 

 to the length of the fibrilla, the fibrilla is said to contract, in which case it decreases in length. The terms relaxa- 

 tion and contraction, when so employed, are at once inaccurate and misleading. Contraction, strictly speaking, 

 implies a diminution in bulk which does not occur when the fibrilla shortens. The sarcous elements are at present 

 (and contrary to observation) accredited with only one power, namely, the power of contracting or shortening the 

 fibrilla in the direction of its length ; the power of elongating the fibrilla in the direction of its length being denied. 

 The sarcous elements, according to the mechanical school, are invested with a centripetal, shortening, or closing 

 power, but are said to have no centrifugal, elongating, or opening power. They aver that the closing or contracting 

 power is traceable to the operation of hfe, but that the dilating or elongating power can only be ascribed to elas- 

 ticity. This theory of muscular action is ahke at variance with the appearances seen under the microscope, and 



