324 CHROMOSOMES IN HEREDITY : PHYSIOLOGICAL 



(i) The parts of the chromosome complement have specific and 

 different properties ; this is the principle of differentiation. 



(ii) These parts co-operate in the developmental processes of the 

 organism, so that every part of the chromosome complement is 

 necessary for the normal development of every part of the organism ; 

 this is the principle of co-operation, which is an a posteriori statement 

 of the a priori doctrine of epigenesis. 



Now when these two principles are considered in relation to 

 adaptation they give it a new character. The adaptation of an 

 organism (gamete or zygote) must be held to consist in an 



14 15 16 17 18 19 20 21 22 23 24 25 26 27 

 Number of chromosomes (2n) 



Fig. 103. — Relative fertility of plants with different chromosome 

 members derived from the cross Avena strigosa X A. harhata 

 (,r = 7). (Nishiyama, 1934.) 



adjustment of the proportions of the differentiated particles (genes) 

 to co-operation with one another in joint reaction to external 

 conditions. This adjustment has been described by Bridges (1922) 

 as balance. The " normal " or successful type of gamete or zygote 

 in a species is said to be " balanced " while the types with their 

 constituent elements not so proportioned are said to be 

 " unbalanced." 



Consider a chromosome complement consisting of n qualita- 

 tively differentiated particles or genes. On Bridges' principle the 

 properties of the organism depend on the proportions, i.e., 



interactions, of these particles. There are 



n(n — i) 



such specific 



