May, 1915.] The Chi'omosome Mechanism. 513 



the expression of a character from a specific factor is one which 

 presents a marvelous field for investigation and experimentation. 

 The influence of the ordinary ecological factors has been studied 

 to some extent but not from the exact standpoint of the systematist 

 and geneticist. One need only consider the remarkable structures 

 developed in certain insect galls to be impressed with the fact, that 

 specific characters can be developed without any previous phylog- 

 eny of the character in relation to the factor being involved. It 

 is evident that the same factors may give rise to very diverse types 

 of characters, when their immediate environment is changed. 

 The influence of the sexual condition and one factor or set of fac- 

 tors on another may come under the same general category of 

 environmental influences determining expression. 



On the hypothesis that the chromosomes contain the heredi- 

 tary factors, the possible number of gametes and zygote combina- 

 tions, giving rise to diploid individuals is given below. These results 

 must necessarily follow according to the law of chance so long as 

 the chromosomes retain their individuality, pair as synaptic mates 

 in reduction, and segregate and combine according to the law of 

 probablity. 



Let x = number of chromosomes. 



If x = 1 and 2x = 2 ; 



And chromosomes a ♦ — ♦ n 



f eggs 

 Then gametes <^ or > = a n 



[ spemis 



Possible combinations = 4. 

 aa an na nn 



Hereditary constitutions = 3 

 a2 an n2 



Possible combinations = 16, as follows: 



abab aoab nbab noab 



abao aoao nbao noao 



abnb aonb nbnb nonb 



abno aono nbno nono 



