THEORIES OF HEREDITY 51 



While there seems to be nothing wonderful in the fact that 

 the two daughter- cells of a divided mother-organism should 

 display the same nature and constitution as the original 

 mother-cell, one of the most puzzling problems of biology 

 has been : Why does the germ-cell reproduce the parent ? 



It is this which is the central question involved in the 

 problem of heredity, and to which, as we shall see presently, 

 various answers have been given. 



Omitting altogether the old metaphysical attempts at a 

 solution, we may choose as representatives of the modern 

 scientific ideas the four following : 



(a) Herbert Spencer's theory of physiological units. 



(b) Charles Darwin's theory of Pangenesis. 



(c) Francis Galton's Stirp theory. 



(d) Weismann's Germ-plasm theory. 



We shall be able to trace a certain historical and logical 

 sequence in these theories, which will become apparent on 

 closer examination of them. 



(a) Spencer's Physiological Units. 



To Spencer belongs the honour of having conceived the 

 fruitful idea of organic units, by means of which he was 

 enabled in his Principles of Biology (1864) to put 

 forward the first thoroughgoing scientific theory of heredity. 

 Starting from the fact that regeneration is a common 

 occurrence among living organisms, he tried to explain the 

 reparation of lost tissues or organs by the assumption that 

 the remaining units of the body have a natural tendency 

 to arrange themselves into the shape and form of the 

 missing part. Further, taking the Begonia leaf, which is 

 capable of growing a new plant from the tiniest fragment, 

 he instances this as likewise leading to the same idea— that 

 ** the active units composing a plant or animal of any 

 species have an intrinsic aptitude to aggregate into the 

 form of that species." The units composing the whole 



