EVOLUTION OF ZOOCECIDIA 
B. W. WELLS 
(WITH PLATES XXI, XXII) 
The basic conceptions back of the present treatment of the 
zoocecidial problem are those connoted by the word evolution. 
For some time the idea that galls have had their phylogeny along 
with other organisms and parts of organisms has occupied a promi- 
nent place in the mind of the writer. Definite attention to this 
phase of the zoocecidial problem has resulted in the development of 
outlines of the probable evolution in the various gall groups. 
This paper is an attempt to give in outline merely the phylogeny 
of zoocecidia, based in lesser part on embryological data, in greater 
part on comparative morphological material. Paleontological 
research has brought so few forms to light that no assistance can 
be gained from that source. A discussion of the conceptions 
which are basic for such a treatment of zoocecidia is also given, 
together with the elucidation of a new interpretation pertaining 
to the relation of the lower and higher galls. 
Historical 
In very few papers dealing with the comparative morphological 
aspects of galls has any attention been devoted to phylogenetic 
problems; the emphasis has been on the delineation of anatomical 
detail. No definite studies from a strictly phylogenetic point of 
view seem to have been made by Europeans. Kister’s recent and 
extensive studies (9, 10) do not include this point of view, at 
least not as gall phylogeny is interpreted by the writer. 
' At the outset careful distinction must be made between con- 
ceptions which are basic for the development of phylogenetic out- 
lines and the phylogenetic outlines or “trees”? themselves. His- 
torically two interpretations which are basic for the development 
of zoocecidial phylogenies are extant: (1) gall characters are 
merely the reappearance of the host plant characters, either those 
immediately present or those hypothetically latent in the germ 
Botanical Gazette, vol. 71] [358 
