32 PATTEN. [VoL. XII. 
those shown in Figs. 10, 11, or 65 could not be due to pres- 
sure of membranes or of adjacent appendages. 
It seems to me, therefore, that the immediate cause is a 
local, internal one, independent, to the same extent as the 
normal growths, of external conditions, and having its source 
in some remote instability of the innermost mechanism. 
This class of variations may therefore be called xormal vari- 
ations, to distinguish them from those due to the more imme- 
diate action of the environment. They must be regarded as 
necessary incidents of a particular structure and likely to occur 
in a certain percentage of cases, irrespective of the immediate 
environment. 
In vital processes, what may be at one time or place an inci- 
dental and occasional phenomenon, may become elsewhere 
under other conditions a constantly recurring result. It is 
therefore clear that in estimating relationships by means of 
morphological characters, such variations deserve careful con- 
sideration. They are as likely to throw light on phyllogenetic 
problems as ontogeny. The latter indicates the established 
paths connecting the present with the past; the study of 
normal variations shows us possible paths leading out of the 
present into the future. 
These facts, then, are interesting morphologically in two ways: 
(1) They may be regarded as forming an indirect confirma- 
tion of the view that the lung-books of scorpions and spiders, 
as claimed by Lankester and Kingsley, are invaginated gill- 
bearing appendages, modified for breathing air. 
(2) I have maintained in a former paper on the “Origin 
of Vertebrates from Arachnids,” that the complete visceral 
arches of vertebrates are homologous with the appendages of 
an arachnid-like ancestor, because in Limulus and scorpions, 
the number of these appendages, their innervation, the position 
of their important sense organs, the structure of the meso- 
blastic cavities associated with them, and the nature of the 
muscles arising from these cavities, resemble as a whole the 
corresponding structures in vertebrates. 
Aside from other considerations, the striking difference 
between arthropod appendages and the gill arches of verte- 
