172 THE PROBLEMS OF EVOLUTION 



we may call end-product one. Then end-products 

 two, three, and four, respectively, are formed by 

 the successive additions of a single molecule of 

 alcohol to separate molecules of a part of the im- 

 mediately preceding end-products. The fifth end- 

 product is not so directly related to the others. 

 It is elaborated chiefly through the interaction of 

 hydrocyanic acid and water, but this interaction 

 cannot take place until water is released through 

 dehydration of some of the other compounds. 

 There is little doubt that such sequential reactions 

 as these may be taken as simple models of what 

 goes on on a tremendous scale in the developing 

 germ cell." ^4 I include Guyer's conclusion in 

 order that his use of the illustration may not be 

 misunderstood. The same sequence is an equally 

 valuable illustration of contingent reaction as it 

 may occur during the formation of the gametes 

 in the parental body. Even recognizing the many 

 safeguards which tend to maintain the genes un- 

 modified from generation to generation, a chemical 

 basis for their qualities must exist, and any such 

 basis must be susceptible to change if the proper 

 conditions reach it. 



But this brings us again very close to the pos- 

 sible action of individual responses. If a given 

 condition of environment should prevail for a long 

 period of time, admittedly all individuals of a 



24 Am. Nat., Vol. XLV. p. 285, 1911. 



