96 Studies in the Theory of Descent. 



genesis and not propagation by nursing, 12 As has 

 already been said, however, I would attribute no 

 fundamental importance to the criterion of agamic 

 reproduction the more especially because we are 

 ignorant of the physiological significance of the 

 two modes of propagation; and further, because this 

 principle of classification is entirely external, and 

 only valuable in so far as no better one can be 

 substituted for it. A separation of the modes of 

 cyclical propagation according to their genesis ap- 

 pears to me especially if practicable not alone 

 to be of greater value, but the only correct one, 

 and for this the knowledge of the origin of sea-, 

 sonal dimorphism seems to me to furnish a possi- 

 ble method. 



If, as was indicated above, we designate as 

 metagenesis (in the narrow sense) all those cases in 

 which it must be admitted that a series of dif- 

 ferently aged phyletic stages have furnished the 

 points of departure, and as heterogenesis those 

 cases in which similar phyletic stages have been 

 compelled to produce a cycle of generations by the 

 periodic action of external influences, it is clear 

 that the scope of heterogenesis is by this means 

 considerably extended, and at the same time 

 sharply and precisely defined. 



Under heterogenesis then is comprised, not only 



12 See my memoir, " Uber Bau und Lebenserscheinungen der 

 Leptodora hyalina" Zeitschrift f. wiss. Zool., vol. xxiv. part 3, 

 1874. 



