The Periodicity of Progressive Mutations. 657 
instances of such groups. In such cases STANDFUSS, in 
conducting his well-known experimental investigations 
into the relations between closely allied species of insects, 
uses the expression, "changes like successive explosions." 
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 species. The 
genus is obviously only this original or collective spe- 
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 the picture. 
So much then for the speculations to which an affirma- 
tive answer to the question proposed above (p. 652) 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 to the same question. 
Such a negative answer would imply the assumption 
that all the ancestors of our Oenothera, back to the first 
1 M. STANDFUSS. Experimentelle zoologische Studicn. Neue 
Denkschriften d. allg. schweiz. Ges. f. d. ges. Naturw., 1898, p. 23. 
Further the articles of the same author in The Entomologist, May 
1895, and in Bull. Soc. entomologlqiie dc France, 1901, No. 4. Also 
his Handbuch dcr palaarktischen Grossschmcttcrlinge, Zurich, 1896. 
