72 INVASION 



organiche, ammettere che la stessa identica species possa 

 essersi attuata e concretata in tempi diversi, e in due o pili 

 luoghi distinti, 6 tale improbability, che confina coll' assurdo. 

 Se ogni specie deve ritenersi come il prodotto d'un immenso 

 numero di fenomeni antecedenti, eome I'effeuo di una quasi 

 infinita quantity, di adattamenti ad un ambiente infinitamente 

 mutevole, come pu6 un cosifattamente complicato prodotto 

 essersi ripetuto e manifestato due o piii volte, in due o pili 

 tempi distinti, in due o piii luoghi separati? Una cotal 

 congettura accettata da Grisebach e da altri, 6 per me 

 assurda. lo mi ribello contro essa". 



The idea of polyphylesis, as advanced by Engler, con bains 

 two distinct concepts : (1) that a species may arise in two 

 different places or at two different times from the same 

 species, and (2) that a genus or higher group may arise at 

 different places or times by the convergence of two or more 

 lines of origin. It is here proposed to restrict polyphylesis, 

 as its meaning would indicate, to the second concept, and to 

 employ for the first the term polygenesis, first suggested 

 by Huxley in the sense of polyphylesis. The term polyphy- 

 lesis is extended, however, to cover the origin of those 

 species which arise at different places or times from the 

 convergence of two or more different species, a logical exten- 

 sion of the idea underlying polyphyletic genera, though it 

 may seem at first thought to be absurd. Polygenesis may be 

 formally defined as the origin of one species from another 

 species at two or more distinct places on the earth's surface, 

 at the same time or at different times, or its origin in the 

 same place at different times. Polyphylesis, on the 

 contrary, is the origin of one species from two or more dif- 

 ferent species at different places, at the same time or at 

 different times. It is evident that what is true of species in 

 this connection will hold equally well of genera and higher 

 groups. Opposed to polygenesis is monogenesis, in which a 

 species arises but once from another species ; with polyphy- 

 lesis is to be contrasted monophylesis, in which the soecies 

 arises from a single other species. It will be noticed at 

 once that these two concepts are closely related. The 



