14 
BULLETIN OF WISCONSIN NATURAL HISTORY SOCIETY. VOL. 5, NO. 1. 
damental cross-bars,''' wliich appear to underlie all avian color- 
marks even the "universal avian character" before mentioned, and 
its latest derivatives. 
The passage from the juvenal to the adult plumage often pre- 
sents us with two or more quite strongly contrasted color-patterns. 
The mutationist might see in this a striking illustration of dis- 
continuity in the evolution of species. 
In the normal course of events, the gap is often very wide 
between two successive patterns, but closer study and experiment 
teach us that, while the visible stages may seem entirely discon- 
nected, they are in reality genetically bound together by a contin- 
uous differentiating process, or a system of processes. In most 
cases, perhaps in all, the apparent discontinuity in the phenomena 
means nothing more than a discontinuity in our knowledge. 
Several other specific characters, usually regarded as "sports" 
or "mutations" were considered, and the continuity of their de- 
velopment from minute incipient stages are clearly established. 
*The discovery of the universality of this character and its far- 
reaching significance as an initial foundation for numerous specific 
characters, suggested the need of a thorough investigation to 
determine its nature and mode of development. This task has 
been undertaken by Mr. Oscar Riddle, who has not only reached 
a physiological explanation of the character, but has fully confirmed 
and established the following anticipatian I ventured to make in 1903: 
"Allowing that the feathers of the common pigeon get their full 
length in four weeks, and that the terminal half of the feather (on 
which the bars are distinct enough to be counted) is formed in four- 
teen days, it is found that flir nuiiihcr of bars corrcspoinls iieftrh/ to 
the number of (lays of (jroirth. If this be so, then the bars would be 
zones of daily growth (light = day: dark =: night, or vice versa)."' 
