224 HISTORY OF BOTANY 



changes they underwent in cell division, it was but natural 

 that the chromosomes should be regarded as in some way 

 connected with the transmission. Each germ cell also was 

 seen to contain the same number of chromosomes one- 

 half the number present in the nuclei of the adult so that 

 each parent contributed the same number of " bearers 

 of parental characters " to the nucleus of the zygote. 



I need not go into the theories advanced by earlier 

 writers on this interesting subject ; all of them were 

 essentially based on the idea that the various organs 

 of the body contributed certain units which were accumu- 

 lated in the germ cells, ready to reproduce in the embryo 

 all the structures of the parents from which they were 

 derived. This conception, under the name of " pan- 

 genesis/ 1 was elaborated most fully by Darwin in 1868, 

 who spoke of these hypothetical units as " gemmules." 

 Darwin's theories were sharply criticised in the succeeding 

 years by Galton, Brooks, De Vries, and others, but we 

 need not spend time over these suggested modifications 

 of pangenesis in view of the hypothesis put forward 

 by Weismann in his work on The Germ Plasm in 1885. 

 One conclusion in Galton's work should, however, be 

 referred to, viz. the so-called " law of ancestral heredity." 

 According to this law, if the total inheritance transmitted 

 to an individual be regarded as unity, then one-half is 

 derived from the two immediate parents, one-quarter 

 from the four grand-parents, one-eighth from the eight 

 great-grand-parents, one-sixteenth from the sixteen great- 

 great-grand-parents, and so on. This statistical result 

 was obtained by Galton from a study of the transmission 

 of colour in the fur of a breed of hounds. 



The hypothesis usually known as " Weismann's 

 theory of heredity " was really enunciated ten years 

 previously by Jaeger, who wrote, " through a great 

 series of generations the germinal protoplasm retains its 

 specific properties, dividing in development into a portion 

 out of which the individual is built up, and a portion which 



