The Periodicity of Progressive Mutations. 657 



instances of such groups. In such cases Standkuss, in 

 conducting his well-known experimental investigations 

 into the relations between closely allied species of insects, 

 uses the expression, "changes like successive exphjsiuns."* 

 Every genus rich in species gives him the impression of 

 an explosion. It looks as if an original species burst into 

 hundreds of forms, the smaller species, among which 

 some survived and constitute the present s])ecies. The 

 genus is obviously only this original or collective s\yt- 

 cies. 



Our Fig. 149 could be continued further downwards. 

 From the elementary species we came to the collective 

 species, and from these to the sub-genera and genera ; 

 in the same way to the more remote explosions would 

 correspond the sub-families and families and the higher 

 grades of the system. If the whole system were per- 

 fectly known to us, and if the pedigree had the form of 

 an ordinary dichotomous table, each point of division 

 would represent a period of mutation, from which, how- 

 ever, only the selected lateral branches, and not all those 

 which had arisen, would be included in tlie ])icture. 



So much then for the speculations to which an affirma- 

 tive answer to the question proposed above ( p. 6S2) would 

 lead. In the following section we shall see how naturally 

 these fit in with the results of paleontological investiga- 

 tion ; but we must now discuss what are the results which 

 would follow a negative answer t(^ the same question. 



Such a negative answer would im])ly the assumption 

 that all the ancestors of our Oenothera, back to the first 



^ M. Standfuss. Ext^cyimcnlcUc ::oologischc Studicn. Ncne 

 Denkschriften d. allg. sclnvci/c. Ges. f. cl._ pes. Xatiirw.. iS(>^. p. 2.V 

 Further the articles of tlie same author in The Eutomolojiist. May 

 1.S05, and in BuU. Soc. cntoinolopqnc dc Francr, iQOr. No. 4. .Mso 

 his Handbuch dcr paUuiyktischcn Grosssclunctfrrlini^r. Zurich. iS<X>. 



