No. I.] ORGANIC VARIATION. 305 
these freshwater species possess almost invariably four large 
eyes, the freshwater forms, on the contrary, have a larger 
number of eyes, varying from four to eight, which are also 
smaller than those of the marine species. How is this vari- 
ability in the number of eyes of the freshwater forms to be 
explained ? Now, variability is engendered, in our view, indirectly 
by change of environment ; and we know that the species in 
question have changed their environment by migrating from 
bodies of salt water into freshwater rivers and lakes. (Or, 
in certain cases, they are inhabitants of lakes which were origi- 
nally of marine character, but have become fresh.) The num- 
ber of the eyes of the marine species being very stable, we 
conclude : (1) that the correlation of the organs in the marine 
forms is comparatively complete, and that, therefore, they are 
well adapted to their environment ; and (2) that no variability 
being perceptible, they are neither at present giving rise to new 
species, nor are they themselves of recent origin. On the other 
hand, the eyes of the freshwater forms being very variable in 
number, we conclude for these : (1) that the correlation of their 
organs is not perfect, and hence that they are not fully adapted 
to their environment ; (2) that this variability must have been 
caused by a change of environment within a comparatively 
recent period of time, since the variability is still continuing ; 
and (3), that as the numerical variation of the eyes is above the 
normal four (the number 6-8 being in fact more frequent than 
4 or 5), the species is tending to evolve a form with a greater 
number of eyes than its ancestors possessed (progressive 
numerical development in relation to the eyes). 
A similar case occurs to me in regard to the individual varia- 
tion in number in the rectrices (stiff tail-feathers), of certain 
North American species of grouse. . Table XXI (placed at 
the end of the paper) expresses this variation for Ceztrocercus 
urophasianus, and for Dendrapagus obscurus with its two 
geographical races.!_ According to Mr. Clark’s observations, 
these are the only North American species of grouse evincing 
such variation ; the number of rectrices being stable in Den- 
11 am indebted to the kindness of my friend Mr. Hubert Lyman Clark, of 
Baltimore, for the communication of the facts embodied in Table X XI. 
