EVOLUTION 979 



somes, and that each chromosome is split up the middle longitudin- 

 ally in every typical cell- division, so that each daughter cell receives 

 a longitudinal half of each chromosome. In the fertilisation of an 

 egg-cell by a sperm-cell, analogous chromosomes of maternal and 

 paternal origin are closely apposed in pairs ; and when the fertilised 

 egg-cell divides into two, each daughter-cell receives the normal 

 number of chromosomes, half maternal, half paternal in origin. In 

 this respect, as Huxley said long ago, the web of the future organism 

 has its warp and woof intimately maternal and paternal, though it 



2 



5 



8 



6 



7 



9 



10 



1^ 



Fig. i66. 



Diagram of the Position of the Hereditary Factors or Genes on a Chromosome 

 of the Fruit-fly (Drosophila) . After Morgan. The figures 1-15 indicate 

 the position of as many different genes. 



does not by any means follow that each thread will find equal 

 expression in development. 



But what are genes} The last long answer to this question is to 

 be found in Prof. T. H. Morgan's Silliman Lectures, The Theory of 

 the Gene (1926), a volume of over three hundred pages. Genes or 

 hereditary "factors" are paired elements lying in linked groups in 

 the germinal material. In all probability, they normally lie in a 

 paired linear order in the chromosomes. They are the germinal 

 representatives or initiatives of the heritable characters of the 

 future organism; but one gene may affect several characters, and 

 one character may require the co-operation of many genes. When 



