66 Studies in the Theory of Descent. 



alternatingly influence a series of generations, a 

 cycle is produced in which the changes are 

 transmitted only to those generations which are 

 acted upon by corresponding conditions, and not 

 to the intermediate ones. Characters which have 

 arisen by the action of a summer climate are inhe- 

 rited by the summer generation only, whilst they 

 remain latent in the winter generation. It is the 

 same as with the mandibles of a caterpillar which 

 are latent in the butterfly, and again make their 

 appearance in the corresponding (larval) stage of 

 the succeeding generation. This is not mere 

 hypothesis, but the legitimate inference from the 

 facts. If it be admitted that my conception of 

 seasonal dimorphism as a double climatic variation 

 is correct, the law of " cyclical heredity," 2 as I may 

 term it in contradistinction to " homochronic 

 heredity," which relates only to the ontogenetic 

 stages immediately follows. All those cases 

 which come under the designation of ' alternation 

 of generation/ can obviously be referred to cyclical 

 heredity, as will be explained further on. In the 

 one case the successive generations deport them- 



2 I at first thought of designating the two forms of cyclical 

 or homochronic heredity as ontogenetic- and phyletic-cyclical 

 heredity. The former would certainly be correct ; the latter 

 would be also applicable to alternation of generation (in which 

 actually two or more phyletic stages alternate with each other) 

 but not to all those cases which I attribute to heterogenesis, 

 in which, as with seasonal dimorphism, a series of generations 

 of the same phyletic stage constitute the point of departure. 



