46 EVOLUTION OF LIVING ORGANISMS. 



happens in such a case ? For an answer we . must turn 

 to the facts and theories of heredity. 



In his great work, Animals and Plants under Domesti- 

 cation, Darwin brought out the provisional hypothesis 

 of Pangenesis, the first complete corpuscular theory of 

 heredity. He supposed that each cell of an organism 

 gives off a living gemmule or pangene, that the pangenes 

 are collected together in the germ-cells, and that they 

 give rise in the next generation to cells similar to those 

 from which they have been derived. Darwin thus sug- 

 gested a means of transmission both for hereditary factors 

 and for modifications in accordance with the views then 

 prevalent. When Weismann showed that modifications 

 are not transmitted as such, the first part of the hypo- 

 thesis of Pangenesis became unnecessary, and his elabo- 

 rate theory of inheritance by means of corpuscles, the 

 determinants, is an extension of Darwin's theory of the 

 distribution of the pangenes from the germ-cells in 

 development. Each independently variable organ or cell 

 is supposed to be represented in the germ-plasm by a 

 separate determinant, itself compounded of several of 

 the hypothetical ultimate units of life, the biophors. 

 These, and the determinants formed of them, are sup- 

 posed to multiply, and to be transmitted along the con- 

 tinuous stream of germ-cells. Ingenious as is the theory, 

 it depends too much on unproved assumptions to carry 

 conviction ; moreover, when applied in detail it soon 

 lands us in a maze of difficulties from which there appears 

 to be no escape. As we pointed out in the first chapter, 

 life is not the attribute of any one special substance, and 

 the conception of unit biophors is not really tenable. 



Modern theories of the mechanism of heredity, while 

 less comprehensive are more satisfying, because they are 

 based on direct observation and experiment, and depart 

 as little as possible from the conclusions immediately 

 deducible from the evidence. They were founded on the 

 work of Mendel, whose observations and conclusions 



