86 Studies in the Theory of Descent. 



designated as heterogenesis, and a knowledge of 

 its mode of origination must therefore throw light 

 on the nature and origin of heterogenesis in general. 

 In seasonal dimorphism, as I have attempted to 

 show, it is the direct action of climate, and indeed 

 chiefly that of temperature, which brings about the 

 change in some of the generations. Since these 

 generations have been exposed to the alternating 

 influence of the summer and winter temperature 

 a periodical dimorphism has been developed 

 a regular cycle of dissimilar generations. It 

 has already been asserted that the consecutive 

 generations of a species comport themselves with 

 respect to heredity in a manner precisely similar 

 to that of the ontogenetic stages, and at the same 

 time such succeeding generations point out the 

 parallelism between metamorphosis and hetero- 

 genesis. If influences capable of directly or indi- 

 rectly producing changes operate on any particular 

 stage of development, these changes are always 

 transmitted to the same stage. Upon this meta- 

 morphosis depends. In a precisely similar manner 

 changes which operated periodically on certain 

 generations (i, 3, 5, for instance) are transmitted 

 to these generations only, and not to the inter- 

 mediate ones. Upon this depends heterogenesis. 

 We have just been led to the comprehension of 

 heterogenesis by cyclical heredity, by the fact 

 that a cycle is produced whenever a series of gene- 

 rations exists under regularly alternating influences. 



