214 DARWINISM TO-DAY. 



4 Various theories of ultimate protoplasmic structure have been 

 proposed to explain what is not really known about this substance. 

 Theories of ul- These theories refer almost exclusively to the physi- 

 timate protoplas- cal, rather than the chemical, make-up of protoplasm, 

 mic structure, and for the most part have been proposed with 

 special attention to the germ-plasm, i. e., the protoplasm of the sperm- 

 and egg-cells. The spur to the formation of these theories is the 

 necessity that biologists have felt imposed on them from the be- 

 ginning of the study of heredity and development of offering some 

 rational explanation of those phenomena. That from a single germ- 

 cell formed by the fusion of a sperm-cell and an egg-cell from 

 different parents, a complete new organism composed of millions 

 of cells of manifold variety of specialisation and arrangement can 

 develop, is wonder enough ; but that this new organism shall repeat 

 in all its parts with extraordinary fidelity the structure and physi- 

 cal idiosyncrasies of one, or show a combination of the character- 

 istics of both, of the individuals from which came the original 

 single sperm- and single egg-cell, adds wonder to wonder. What 

 physical or structural basis is there in the fertilised egg-cell that it 

 can represent in its tiny self the whole of a giant body, like that of 

 an elephant, whose every detail it can, by a process of development 

 under suitable extrinsic conditions of temperature, food-supply, etc., 

 repeat in a new creature. The answers to this, all purely specula- 

 tive, or more fairly theoretical, because some of the answers at 

 least have been guarded in their forming by all the care which a 

 rigorous scientific attitude toward hypothesis demands, are many 

 and various, and date from the days of the Greek philosophers to 

 the present hour. It would take too much space and carry us too 

 far afield to attempt anything like an explanatory list of even all 

 of the better known of these general theories of the invisible ulti- 

 mate structure of the germ-plasm here, but by selecting seven or 

 eight types of the principal categories or kinds of these theories, 

 and briefly explaining them, we may have at least some conception 

 of the attitude that biologists take toward this great problem. The 

 reader who has a fancy for following this subject further is re- 

 ferred to the admirably full and lucid treatment of it in Delage's 

 great work, "L'Heredite" (2d ed., pp. 431-772, 1903). 



Most of these theories include much more in them than a simple 

 speculation as to the ultimate structure of the life-substance ; they 

 attempt to explain all the phenomena of life, motion, nutrition, 

 growth, reproduction, development, heredity, variation, etc., with 

 reference to some assumed ultimate make-up of the primitive life- 

 substance, and the relation of this structure to the known physico- 

 chemical forces and conditions of Nature. Most of the older 



